CSS Guidelines is a document by me, Harry Roberts. I am a Consultant Front-end Architect from the UK, and I help companies all over the world write and manage better quality UIs for their products and teams. I am available for hire.
CSS Guidelines is provided through a pay-what-you-like model—from $0 upward. If CSS Guidelines is useful to you or your team, please consider supporting it.
Get updates about changes, additions, and new and upcoming sections by following @cssguidelines on Twitter.
CSS 是一门容易学习和上手的语言，但它并不完美。 我们并不能改变 CSS 如何工作，但可以改变编写 CSS 代码的习惯，让它变得更美好。
在大型项目中，你会与不同类型和能力的工程师协作，为了保持代码的一致性需要完成以下几点：
为了满足以上目标，我们需要应用多种技巧，而 CSS Guideline 是最重要的技巧之一。
编码风格指南（注意，不止是视觉风格指南）对于以下这样的团队是有价值的
优秀的编码风格指南，需要满足以下几点：
CSS guideline 不仅仅是一份编码风格指南，它包含一套方法和技巧，是经过多年实战尝试，考验和重构所提取出来的精华。 我强力地推荐给我的客户和团队，你也可以根据自身需求进行调整。
CSS is not a pretty language While it is simple to learn and get started with, it soon becomes problematic at any reasonable scale. There isn’t much we can do to change how CSS works, but we can make changes to the way we author and structure it.
In working on large, long-running projects, with dozens of developers of differing specialities and abilities, it is important that we all work in a unified way in order to—among other things—
There are a variety of techniques we must employ in order to satisfy these goals, and CSS Guidelines is a document of recommendations and approaches that will help us to do so.
A coding styleguide (note, not a visual styleguide) is a valuable tool for teams who
Whilst styleguides are typically more suited to product teams—large codebases on long-lived and evolving projects, with multiple developers contributing over prolonged periods of time—all developers should strive for a degree of standardisation in their code.
A good styleguide, when well followed, will
Styleguides should be learned, understood, and implemented at all times on a project which is governed by one, and any deviation must be fully justified.
CSS Guidelines is a styleguide; it is not the styleguide. It contains methodologies, techniques, and tips that I would firmly recommend to my clients and teams, but your own tastes and circumstances may well be different. Your mileage may vary.
These guidelines are opinionated, but they have been repeatedly tried, tested, stressed, refined, broken, reworked, and revisited over a number of years on projects of all sizes.
One of the simplest forms of a styleguide is a set of rules regarding syntax and formatting. Having a standard way of writing (literally writing) CSS means that code will always look and feel familiar to all members of the team.
Further, code that looks clean feels clean. It is a much nicer environment to work in, and prompts other team members to maintain the standard of cleanliness that they found. Ugly code sets a bad precedent.
At a very high-level, we want
But, as with anything, the specifics are somewhat irrelevant—consistency is key.
With the meteoric rise of preprocessors of late, more often is the case that developers are splitting CSS across multiple files.
Even if not using a preprocessor, it is a good idea to split discrete chunks of code into their own files, which are concatenated during a build step.
If, for whatever reason, you are not working across multiple files, the next sections might require some bending to fit your setup.
A table of contents is a fairly substantial maintenance overhead, but the benefits it brings far outweigh any costs. It takes a diligent developer to keep a table of contents up to date, but it is well worth sticking with. An up-to-date table of contents provides a team with a single, canonical catalogue of what is in a CSS project, what it does, and in what order.
A simple table of contents will—in order, naturally—simply provide the name of the section and a brief summary of what it is and does, for example:
/**
* CONTENTS
*
* SETTINGS
* Global...............Globally-available variables and config.
*
* TOOLS
* Mixins...............Useful mixins.
*
* GENERIC
* Normalize.css........A level playing field.
* Box-sizing...........Better default `box-sizing`.
*
* BASE
* Headings.............H1–H6 styles.
*
* OBJECTS
* Wrappers.............Wrapping and constraining elements.
*
* COMPONENTS
* Page-head............The main page header.
* Page-foot............The main page footer.
* Buttons..............Button elements.
*
* TRUMPS
* Text.................Text helpers.
*/
Each item maps to a section and/or include.
Naturally, this section would be substantially larger on the majority of projects, but hopefully we can see how this section—in the master stylesheet—provides developers with a project-wide view of what is being used where, and why.
Where possible, limit CSS files’ width to 80 characters. Reasons for this include
/**
* I am a long-form comment. I describe, in detail, the CSS that follows. I am
* such a long comment that I easily break the 80 character limit, so I am
* broken across several lines.
*/There will be unavoidable exceptions to this rule—such as URLs, or gradient syntax—which shouldn’t be worried about.
Begin every new major section of a CSS project with a title:
/*------------------------------------*\
#SECTION-TITLE
\*------------------------------------*/
.selector {}
The title of the section is prefixed with a hash (#) symbol to allow us to
perform more targeted searches (e.g. grep, etc.): instead of searching for
just SECTION-TITLE—which may yield many results—a more scoped search
of #SECTION-TITLE should return only the section in question.
Leave a carriage return between this title and the next line of code (be that a comment, some Sass, or some CSS).
If you are working on a project where each section is its own file, this title should appear at the top of each one. If you are working on a project with multiple sections per file, each title should be preceded by five (5) carriage returns. This extra whitespace coupled with a title makes new sections much easier to spot when scrolling through large files:
/*------------------------------------*\
#A-SECTION
\*------------------------------------*/
.selector {}
/*------------------------------------*\
#ANOTHER-SECTION
\*------------------------------------*/
/**
* Comment
*/
.another-selector {}
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Before we discuss how we write out our rulesets, let’s first familiarise ourselves with the relevant terminology:
[selector] {
[property]: [value];
[<--declaration--->]
}
For example:
.foo, .foo--bar,
.baz {
display: block;
background-color: green;
color: red;
}
Here you can see we have
{);:);{) on the same line as our last selector;{);}) on its own new line;;) on our last declaration.This format seems to be the largely universal standard (except for variations in number of spaces, with a lot of developers preferring two (2)).
As such, the following would be incorrect:
.foo, .foo--bar, .baz
{
display:block;
background-color:green;
color:red }
Problems here include
{) on its own line;}) does not sit on its own line;;) is missing;:).CSS should be written across multiple lines, except in very specific circumstances. There are a number of benefits to this:
diffs, because one line only ever carries one
change.Exceptions to this rule should be fairly apparent, such as similar rulesets that only carry one declaration each, for example:
.icon {
display: inline-block;
width: 16px;
height: 16px;
background-image: url(/img/sprite.svg);
}
.icon--home { background-position: 0 0 ; }
.icon--person { background-position: -16px 0 ; }
.icon--files { background-position: 0 -16px; }
.icon--settings { background-position: -16px -16px; }
These types of ruleset benefit from being single-lined because
As well as intending individual declarations, indent entire related rulesets to signal their relation to one another, for example:
.foo {}
.foo__bar {}
.foo__baz {}
By doing this, a developer can see at a glance that .foo__baz {} lives inside
.foo__bar {} lives inside .foo {}.
This quasi-replication of the DOM tells developers a lot about where classes are expected to be used without them having to refer to a snippet of HTML.
Sass provides nesting functionality. That is to say, by writing this:
.foo {
color: red;
.bar {
color: blue;
}
}
…we will be left with this compiled CSS:
.foo { color: red; }
.foo .bar { color: blue; }
When indenting Sass, we stick to the same four (4) spaces, and we also leave a blank line before and after the nested ruleset.
N.B. Nesting in Sass should be avoided wherever possible. See the Specificity section for more details.
Attempt to align common and related identical strings in declarations, for example:
.foo {
-webkit-border-radius: 3px;
-moz-border-radius: 3px;
border-radius: 3px;
}
.bar {
position: absolute;
top: 0;
right: 0;
bottom: 0;
left: 0;
margin-right: -10px;
margin-left: -10px;
padding-right: 10px;
padding-left: 10px;
}
This makes life a little easier for developers whose text editors support column editing, allowing them to change several identical and aligned lines in one go.
As well as indentation, we can provide a lot of information through liberal and judicious use of whitespace between rulesets. We use:
For example:
/*------------------------------------*\
#FOO
\*------------------------------------*/
.foo {}
.foo__bar {}
.foo--baz {}
/*------------------------------------*\
#BAR
\*------------------------------------*/
.bar {}
.bar__baz {}
.bar__foo {}
There should never be a scenario in which two rulesets do not have an empty line between them. This would be incorrect:
.foo {}
.foo__bar {}
.foo--baz {}
Given HTML and CSS’ inherently interconnected nature, it would be remiss of me to not cover some syntax and formatting guidelines for markup.
Always quote attributes, even if they would work without. This reduces the chance of accidents, and is a more familiar format to the majority of developers. For all this would work (and is valid):
<div class=box>
…this format is preferred:
<div class="box">
The quotes are not required here, but err on the safe side and include them.
When writing multiple values in a class attribute, separate them with two spaces, thus:
<div class="foo bar">
When multiple classes are related to each other, consider grouping them in
square brackets ([ and ]), like so:
<div class="[ box box--highlight ] [ bio bio--long ]">
This is not a firm recommendation, and is something I am still trialling myself, but it does carry a number of benefits. Read more in Grouping related classes in your markup.
As with our rulesets, it is possible to use meaningful whitespace in your HTML. You can denote thematic breaks in content with five (5) empty lines, for example:
<header class="page-head">
...
</header>
<main class="page-content">
...
</main>
<footer class="page-foot">
...
</footer>
Something you need some more help with?
Separate independent but loosely related snippets of markup with a single empty line, for example:
<ul class="primary-nav">
<li class="primary-nav__item">
<a href="/" class="primary-nav__link">Home</a>
</li>
<li class="primary-nav__item primary-nav__trigger">
<a href="/about" class="primary-nav__link">About</a>
<ul class="primary-nav__sub-nav">
<li><a href="/about/products">Products</a></li>
<li><a href="/about/company">Company</a></li>
</ul>
</li>
<li class="primary-nav__item">
<a href="/contact" class="primary-nav__link">Contact</a>
</li>
</ul>
This allows developers to spot separate parts of the DOM at a glance, and also allows certain text editors—like Vim, for example—to manipulate empty-line-delimited blocks of markup.
The cognitive overhead of working with CSS is huge. With so much to be aware of, and so many project-specific nuances to remember, the worst situation most developers find themselves in is being the-person-who-didn’t-write-this-code. Remembering your own classes, rules, objects, and helpers is manageable to an extent, but anyone inheriting CSS barely stands a chance.
CSS needs more comments.
As CSS is something of a declarative language that doesn’t really leave much of a paper-trail, it is often hard to discern—from looking at the CSS alone—
This doesn’t even take into account some of CSS’ many quirks—such as various
sates of overflow triggering block formatting context, or certain transform
properties triggering hardware acceleration—that make it even more baffling to
developers inheriting projects.
As a result of CSS not telling its own story very well, it is a language that really does benefit from being heavily commented.
As a rule, you should comment anything that isn’t immediately obvious from the
code alone. That is to say, there is no need to tell someone that color: red;
will make something red, but if you’re using overflow: hidden; to clear
floats—as opposed to clipping an element’s overflow—this is probably something
worth documenting.
For large comments that document entire sections or components, we use a DocBlock-esque multi-line comment which adheres to our 80 column width.
Here is a real-life example from the CSS which styles the page header on CSS Wizardry:
/**
* The site’s main page-head can have two different states:
*
* 1) Regular page-head with no backgrounds or extra treatments; it just
* contains the logo and nav.
* 2) A masthead that has a fluid-height (becoming fixed after a certain point)
* which has a large background image, and some supporting text.
*
* The regular page-head is incredibly simple, but the masthead version has some
* slightly intermingled dependency with the wrapper that lives inside it.
*/
This level of detail should be the norm for all non-trivial code—descriptions of states, permutations, conditions, and treatments.
When working across multiple partials, or in an OOCSS manner, you will often find that rulesets that can work in conjunction with each other are not always in the same file or location. For example, you may have a generic button object—which provides purely structural styles—which is to be extended in a component-level partial which will add cosmetics. We document this relationship across files with simple object–extension pointers. In the object file:
/**
* Extend `.btn {}` in _components.buttons.scss.
*/
.btn {}
And in your theme file:
/**
* These rules extend `.btn {}` in _objects.buttons.scss.
*/
.btn--positive {}
.btn--negative {}
This simple, low effort commenting can make a lot of difference to developers who are unaware of relationships across projects, or who are wanting to know how, why, and where other styles might be being inherited from.
Oftentimes we want to comment on specific declarations (i.e. lines) in a ruleset. To do this we use a kind of reverse footnote. Here is a more complex comment detailing the larger site headers mentioned above:
/**
* Large site headers act more like mastheads. They have a faux-fluid-height
* which is controlled by the wrapping element inside it.
*
* 1. Mastheads will typically have dark backgrounds, so we need to make sure
* the contrast is okay. This value is subject to change as the background
* image changes.
* 2. We need to delegate a lot of the masthead’s layout to its wrapper element
* rather than the masthead itself: it is to this wrapper that most things
* are positioned.
* 3. The wrapper needs positioning context for us to lay our nav and masthead
* text in.
* 4. Faux-fluid-height technique: simply create the illusion of fluid height by
* creating space via a percentage padding, and then position everything over
* the top of that. This percentage gives us a 16:9 ratio.
* 5. When the viewport is at 758px wide, our 16:9 ratio means that the masthead
* is currently rendered at 480px high. Let’s…
* 6. …seamlessly snip off the fluid feature at this height, and…
* 7. …fix the height at 480px. This means that we should see no jumps in height
* as the masthead moves from fluid to fixed. This actual value takes into
* account the padding and the top border on the header itself.
*/
.page-head--masthead {
margin-bottom: 0;
background: url(/img/css/masthead.jpg) center center #2e2620;
@include vendor(background-size, cover);
color: $color-masthead; /* [1] */
border-top-color: $color-masthead;
border-bottom-width: 0;
box-shadow: 0 0 10px rgba(0, 0, 0, 0.1) inset;
@include media-query(lap-and-up) {
background-image: url(/img/css/masthead-medium.jpg);
}
@include media-query(desk) {
background-image: url(/img/css/masthead-large.jpg);
}
> .wrapper { /* [2] */
position: relative; /* [3] */
padding-top: 56.25%; /* [4] */
@media screen and (min-width: 758px) { /* [5] */
padding-top: 0; /* [6] */
height: $header-max-height - double($spacing-unit) - $header-border-width; /* [7] */
}
}
}
These types of comment allow us to keep all of our documentation in one place whilst referring to the parts of the ruleset to which they belong.
With most—if not all—preprocessors, we have the option to write comments that will not get compiled out into our resulting CSS file. As a rule, use these comments to document code that would not get written out to that CSS file either. If you are documenting code which will get compiled, use comments that will compile also. For example, this is correct:
// Dimensions of the @2x image sprite:
$sprite-width: 920px;
$sprite-height: 212px;
/**
* 1. Default icon size is 16px.
* 2. Squash down the retina sprite to display at the correct size.
*/
.sprite {
width: 16px; /* [1] */
height: 16px; /* [1] */
background-image: url(/img/sprites/main.png);
background-size: ($sprite-width / 2 ) ($sprite-height / 2); /* [2] */
}
We have documented variables—code which will not get compiled into our CSS file—with preprocessor comments, whereas our CSS—code which will get compiled into our CSS file—is documented using CSS comments. This means that we have only the correct and relevant information available to us when debugging our compiled stylesheets.
It should go without saying that no comments should make their way into production environments—all CSS should be minified, resulting in loss of comments, before being deployed.
Naming conventions in CSS are hugely useful in making your code more strict, more transparent, and more informative.
A good naming convention will tell you and your team
The naming convention I follow is very simple: hyphen (-) delimited strings,
with BEM-like naming for more complex pieces of code.
It’s worth noting that a naming convention is not normally useful CSS-side of development; they really come into their own when viewed in HTML.
All strings in classes are delimited with a hyphen (-), like so:
.page-head {}
.sub-content {}
Camel case and underscores are not used for regular classes; the following are incorrect:
.pageHead {}
.sub_content {}
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For larger, more interrelated pieces of UI that require a number of classes, we use a BEM-like naming convention.
BEM, meaning Block, Element, Modifier, is a front-end methodology coined by developers working at Yandex. Whilst BEM is a complete methodology, here we are only concerned with its naming convention. Further, the naming convention here only is BEM-like; the principles are exactly the same, but the actual syntax differs slightly.
BEM splits components’ classes into three groups:
To take an analogy (note, not an example):
.person {}
.person__head {}
.person--tall {}
Elements are delimited with two (2) underscores (__), and Modifiers are
delimited by two (2) hyphens (--).
Here we can see that .person {} is the Block; it is the sole root of a
discrete entity. .person__head {} is an Element; it is a smaller part of the
.person {} Block. Finally, .person--tall {} is a Modifier; it is a specific
variant of the .person {} Block.
Your Block context starts at the most logical, self-contained, discrete
location. To continue with our person-based analogy, we’d not have a class like
.room__person {}, as the room is another, much higher context. We’d probably
have separate Blocks, like so:
.room {}
.room__door {}
.room--kitchen {}
.person {}
.person__head {}
If we did want to denote a .person {} inside a .room {}, it is more correct
to use a selector like .room .person {} which bridges two Blocks than it is to
increase the scope of existing Blocks and Elements.
A more realistic example of properly scoped blocks might look something like this, where each chunk of code represents its own Block:
.page {}
.content {}
.sub-content {}
.footer {}
.footer__copyright {}
Incorrect notation for this would be:
.page {}
.page__content {}
.page__sub-content {}
.page__footer {}
.page__copyright {}
It is important to know when BEM scope starts and stops. As a rule, BEM applies to self-contained, discrete parts of the UI.
If we were to add another Element—called, let’s say, .person__eye {}—to this
.person {} component, we would not need to step through every layer of the
DOM. That is to say, the correct notation would be .person__eye {}, and not
.person__head__eye {}. Your classes do not reflect the full paper-trail of the
DOM.
You can have variants of Elements, and these can be denoted in a number of ways depending on how and why they are being modified. Carrying on with our person example, a blue eye might look like this:
.person__eye--blue {}
Here we can see we’re directly modifying the eye Element.
Things can get more complex, however. Please excuse the crude analogy, and let’s imagine we have a face Element that is handsome. The person themselves isn’t that handsome, so we modify the face Element directly—a handsome face on a regular person:
.person__face--handsome {}
But what if that person is handsome, and we want to style their face because of that fact? A regular face on a handsome person:
.person--handsome .person__face {}
Here is one of a few occasions where we’d use a descendant selector to modify an Element based on a Modifier on the Block.
If using Sass, we would likely write this like so:
.person {}
.person__face {
.person--handsome & {}
}
.person--handsome {}
Note that we do not nest a new instance of .person__face {} inside of
.person--handsome {}; instead, we make use of Sass’ parent selectors to
prepend .person--handsome onto the existing .person__face {} selector. This
means that all of our .person__face {}-related rules exist in once place, and
aren’t spread throughout the file. This is general good practice when dealing
with nested code: keep all of your context (e.g. all .person__face {} code)
encapsulated in one location.
As I previously hinted at, naming conventions aren’t necessarily all that useful in your CSS. Where naming conventions’ power really lies is in your markup. Take the following, non-naming-conventioned HTML:
<div class="box profile pro-user">
<img class="avatar image" />
<p class="bio">...</p>
</div>
How are the classes box and profile related to each other? How are the
classes profile and avatar related to each other? Are they related at all?
Should you be using pro-user alongside bio? Will the classes image and
profile live in the same part of the CSS? Can you use avatar anywhere else?
From that markup alone, it is very hard to answer any of those questions. Using a naming convention, however, changes all that:
<div class="box profile profile--is-pro-user">
<img class="avatar profile__image" />
<p class="profile__bio">...</p>
</div>
Now we can clearly see which classes are and are not related to each other, and how; we know what classes we can’t use outside of the scope of this component; and we know which classes we may be free to reuse elsewhere.
As a rule, it is unwise to bind your CSS and your JS onto the same class in your HTML. This is because doing so means you can’t have (or remove) one without (removing) the other. It is much cleaner, much more transparent, and much more maintainable to bind your JS onto specific classes.
I have known occasions before when trying to refactor some CSS has unwittingly removed JS functionality because the two were tied to each other—it was impossible to have one without the other.
Typically, these are classes that are prepended with js-, for example:
<input type="submit" class="btn js-btn" value="Follow" />
This means that we can have an element elsewhere which can carry with style of
.btn {}, but without the behaviour of .js-btn.
data-* AttributesA common practice is to use data-* attributes as JS hooks, but this is
incorrect. data-* attributes, as per the spec, are used to store custom
data private to the page or application
(emphasis mine). data-*
attributes are designed to store data, not be bound to.
As previously mentioned, these are very simple naming conventions, and ones that don’t do much more than denote three distinct groups of class.
I would encourage you to read up on and further look in to your naming convention in order to provide more functionality—I know it’s something I’m keen to research and investigate further.
Perhaps somewhat surprisingly, one of the most fundamental, critical aspects of writing maintainable and scalable CSS is selectors. Their specificity, their portability, and their reusability all have a direct impact on the mileage we will get out of our CSS, and the headaches it might bring us.
It is important when writing CSS that we scope our selectors correctly, and that we’re selecting the right things for the right reasons. Selector Intent is the process of deciding and defining what you want to style and how you will go about selecting it. For example, if you are wanting to style your website’s main navigation menu, a selector like this would be incredibly unwise:
header ul {}
This selector’s intent is to style any ul inside any header element, whereas
our intent was to style the site’s main navigation. This is poor Selector
Intent: you can have any number of header elements on a page, and they in turn
can house any number of uls, so a selector like this runs the risk of applying
very specific styling to a very wide number of elements. This will result in
having to write more CSS to undo the greedy nature of such a selector.
A better approach would be a selector like:
.site-nav {}
An unambiguous, explicit selector with good Selector Intent. We are explicitly selecting the right thing for exactly the right reason.
Poor Selector Intent is one of the biggest reasons for headaches on CSS projects. Writing rules that are far too greedy—and that apply very specific treatments via very far reaching selectors—causes unexpected side effects and leads to very tangled stylesheets, with selectors overstepping their intentions and impacting and interfering with otherwise unrelated rulesets.
CSS cannot be encapsulated, it is inherently leaky, but we can mitigate some of these effects by not writing such globally-operating selectors: your selectors should be as explicit and well reasoned as your reason for wanting to select something.
With a move toward a more component-based approach to constructing UIs, the idea of reusability is paramount. We want the option to be able to move, recycle, duplicate, and syndicate components across our projects.
To this end, we make heavy use of classes. IDs, as well as being hugely over-specific, cannot be used more than once on any given page, whereas classes can be reused an infinite amount of times. Everything you choose, from the type of selector to its name, should lend itself toward being reused.
Given the ever-changing nature of most UI projects, and the move to more component-based architectures, it is in our interests not to style things based on where they are, but on what they are. That is to say, our components’ styling should not be reliant upon where we place them—they should remain entirely location independent.
Let’s take an example of a call-to-action button that we have chosen to style via the following selector:
.promo a {}
Not only does this have poor Selector Intent—it will greedily style any and
every link inside of a .promo to look like a button—it is also pretty wasteful
as a result of being so locationally dependent: we can’t reuse that button with
its correct styling outside of .promo because it is explicitly tied to that
location. A far better selector would have been:
.btn {}
This single class can be reused anywhere outside of .promo and will always
carry its correct styling. As a result of a better selector, this piece of UI is
more portable, more recyclable, doesn’t have any dependencies, and has much
better Selector Intent. A component shouldn’t have to live in a certain place
to look a certain way.
Something you need some more help with?
Reducing, or, ideally, removing, location dependence means that we can move components around our markup more freely, but how about improving our ability to move classes around components? On a much lower level, there are changes we can make to our selectors that make the selectors themselves—as opposed to the components they create—more portable. Take the following example:
input.btn {}
This is a qualified selector; the leading input ties this ruleset to
only being able to work on input elements. By omitting this qualification, we
allow ourselves to reuse the .btn class on any element we choose, like an a,
for example, or a button.
Qualified selectors do not lend themselves well to being reused, and every selector we write should be authored with reuse in mind.
Of course, there are times when you may want to legitimately qualify a selector—you might need to apply some very specific styling to a particular element when it carries a certain class, for example:
/**
* Embolden and colour any element with a class of `.error`.
*/
.error {
color: red;
font-weight: bold;
}
/**
* If the element is a `div`, also give it some box-like styling.
*/
div.error {
padding: 10px;
border: 1px solid;
}
This is one example where a qualified selector might be justifiable, but I would still recommend an approach more like:
/**
* Text-level errors.
*/
.error-text {
color: red;
font-weight: bold;
}
/**
* Elements that contain errors.
*/
.error-box {
padding: 10px;
border: 1px solid;
}
This means that we can apply .error-box to any element, and not just a
div—it is more reusable than a qualified selector.
One thing that qualified selectors can be useful for is signalling where a class might be expected or intended to be used, for example:
ul.nav {}
Here we can see that the .nav class is meant to be used on a ul element, and
not on a nav. By using quasi-qualified selectors we can still provide
that information without actually qualifying the selector:
/*ul*/.nav {}
By commenting out the leading element, we can still leave it to be read, but avoid qualifying and increasing the specificity of the selector.
As Phil Karlton once said, There are only two hard things in Computer
Science: cache invalidation and naming things.
I won’t comment on the former claim here, but the latter has plagued me for
years. My advice with regard to naming things in CSS is to pick a name that is
sensible, but somewhat ambiguous: aim for high reusability. For example, instead
of a class like .site-nav, choose something like .primary-nav; rather than
.footer-links, favour a class like .sub-links.
The differences in these names is that the first of each two examples is tied to a very specific use case: they can only be used as the site’s navigation or the footer’s links respectively. By using slightly more ambiguous names, we can increase our ability to reuse these components in different circumstances.
To quote Nicolas Gallagher:
Tying your class name semantics tightly to the nature of the content has already reduced the ability of your architecture to scale or be easily put to use by other developers.
That is to say, we should use sensible names—classes like .border or .red
are never advisable—but we should avoid using classes which describe the exact
nature of the content and/or its use cases. Using a class name to describe
content is redundant because content describes itself.
The debate surrounding semantics has raged for years, but it is important that we adopt a more pragmatic, sensible approach to naming things in order to work more efficiently and effectively. Instead of focussing on ‘semantics’, look more closely at sensibility and longevity—choose names based on ease of maintenance, not for their perceived meaning.
Name things for people; they’re the only things that actually read your classes (everything else merely matches them). Once again, it is better to strive for reusable, recyclable classes rather than writing for specific use cases. Let’s take an example:
/**
* Runs the risk of becoming out of date; not very maintainable.
*/
.blue {
color: blue;
}
/**
* Depends on location in order to be rendered properly.
*/
.header span {
color: blue;
}
/**
* Too specific; limits our ability to reuse.
*/
.header-color {
color: blue;
}
/**
* Nicely abstracted, very portable, doesn’t risk becoming out of date.
*/
.highlight-color {
color: blue;
}
It is important to strike a balance between names that do not literally describe
the style that the class brings, but also ones that do not explicitly describe
specific use cases. Instead of .home-page-panel, choose .masthead; instead
of .site-nav, favour .primary-nav; instead of .btn-login, opt for
.btn-primary.
Naming components with agnosticism and reusability in mind really helps
developers construct and modify UIs much more quickly, and with far less waste.
However, it can sometimes be beneficial to provide more specific or meaningful
naming alongside the more ambiguous class, particularly when several agnostic
classes come together to form a more complex and specific component that might
benefit from having a more meaningful name. In this scenario, we augment the
classes with a data-ui-component attribute which houses a more specific name,
for example:
<ul class="tabbed-nav" data-ui-component="Main Nav">
Here we have the benefits of a highly reusable class name which does not
describe—and, therefore, tie itself to—a specific use case, and added meaning
via our data-ui-component attribute. The data-ui-component’s value can take
any format you wish, like title case:
<ul class="tabbed-nav" data-ui-component="Main Nav">
Or class-like:
<ul class="tabbed-nav" data-ui-component="main-nav">
Or namespaced:
<ul class="tabbed-nav" data-ui-component="nav-main">
The implementation is largely personal preference, but the concept still remains: Add any useful or specific meaning via a mechanism that will not inhibit your and your team’s ability to recycle and reuse CSS.
A topic which is—with the quality of today’s browsers—more interesting than it is important, is selector performance. That is to say, how quickly a browser can match the selectors your write in CSS up with the nodes it finds in the DOM.
Generally speaking, the longer a selector is (i.e. the more component parts) the slower it is, for example:
body.home div.header ul {}
…is a far less efficient selector than:
.primary-nav {}
This is because browsers read CSS selectors right-to-left. A browser will read the first selector as
ul elements in the DOM;.header;.header class exists on a div element;.home;.home exists on a body element.The second, in contrast, is simply a case of the browser reading
.primary-nav.To further compound the problem, we are using descendant selectors (e.g. .foo
.bar {}). The upshot of this is that a browser is required to start with the
rightmost part of the selector (i.e. .bar) and keep looking up the DOM
indefinitely until it finds the next part (i.e. .foo). This could mean
stepping up the DOM dozens of times until a match is found.
This is just one reason why nesting with preprocessors is often a false economy; as well as making selectors unnecessarily more specific, and creating location dependency, it also creates more work for the browser.
By using a child selector (e.g. .foo > .bar {}) we can make the process much
more efficient, because this only requires the browser to look one level higher
in the DOM, and it will stop regardless of whether or not it found a match.
Because browsers read selectors right-to-left, the rightmost selector is often critical in defining a selector’s performance: this is called the key selector.
The following selector might appear to be highly performant at first glance. It uses an ID which is nice and fast, and there can only ever be one on a page, so surely this will be a nice and speedy lookup—just find that one ID and then style everything inside of it:
#foo * {}
The problem with this selector is that the key selector (*) is very, very
far reaching. What this selector actually does is finds every single node in
the DOM (even <title>, <link>, and <head> elements; everything) and then
looks to see if it lives anywhere at any level within #foo. This is a very,
very expensive selector, and should most likely be avoided or rewritten.
Thankfully, by writing selectors with good Selector Intent, we are probably avoiding inefficient selectors by default; we are very unlikely to have greedy key selectors if we’re targeting the right things for the right reason.
That said, however, CSS selector performance should be fairly low on your list of things to optimise; browsers are fast, and are only ever getting faster, and it is only on notable edge cases that inefficient selectors would be likely to pose a problem.
As well as their own specific issues, nesting, qualifying, and poor Selector Intent all contribute to less efficient selectors.
Your selectors are fundamental to writing good CSS. To very briefly sum up the above sections:
Focussing on these points will keep your selectors a lot more sane and easy to work with on changing and long-running projects.