CSS guidelines A developer Must Know

your stylesheets should be maximum 80 character width where possible. Exceptions may be gradient syntax and URLs in comments. That’s fine, there’s nothing we can do about that.

I prefer four (FOUR) space indents over tabs and write multi-line CSS.

One file vs. many files in your stylesheets

Some Designer prefer to work with single, large files. This is ok, and by sticking to the following guidelines you’ll encounter no problems. Since moving to Sass I have started sharding my stylesheets out into lots of tiny includes. This too is fine… Whichever method you choose, the following rules and guidelines apply.

The only notable difference is with regards our table of contents and our section titles.

Table of contents

At the top of stylesheets, I maintain a table of contents which will detail the sections contained in the document, for example:

/*------------------------------------*\
 $CONTENTS
\*------------------------------------*/
/**
 * CONTENTS............You’re reading it!
 * RESET...............Set our reset defaults
 * FONT-FACE...........Import brand font files
 */

This will tell the next developer(s) exactly what they can expect to find in this file. they can under easily . Each item in the table of contents maps directly to a section title.

If you are working in one large stylesheet , the corresponding section will also be in that file. If you are working across multiple files then each item in the table of contents will map to an include which pulls that section in.

Section titles

The table of contents would be of no use unless it had corresponding section titles. Denote a section thus:

/*------------------------------------*\
 $RESET
\*------------------------------------*/

The $ prefixing the name of the section allows us to run a find ([Cmd|Ctrl]+F) for $[SECTION-NAME] and limit our search scope to section titles only.

If you are working in one large stylesheet, you leave five (5) carriage returns between each section, thus:

/*------------------------------------*\
 $RESET
\*------------------------------------*/
[Our
reset
styles]

/*------------------------------------*\
 $FONT-FACE
\*------------------------------------*/

This large chunk of whitespace is quickly noticeable when scrolling quickly through larger files.

If you are working across multiple, included stylesheets, start each of those files with a section title and there is no need for any carriage returns.

Source order

Try and write stylesheets in specificity order. This ensures that you take full advantage of inheritance and CSS’ first C; the cascade.

A well ordered stylesheet will be ordered something like this:

1. Reset – ground zero.


2. Elements – unclassed h1, unclassed ul etc.


3. Objects and abstractions — generic, underlying design patterns.


4. Components – full components constructed from objects and their extensions.


5. Style trumps – error states etc.

This means that—as you go down the document—each section builds upon and inherits sensibly from the previous one(s). There should be less undoing of styles, less specificity problems and all-round better architected stylesheets.

For further reading I cannot recommend Jonathan Snook’s SMACSS highly enough.

Anatomy of rulesets

[selector] 
 [property]: [value];
 [<- Declaration -->]

I have a number of standards when structuring rulesets.

• Use hyphen delimited class names (except for BEM notation, see below)


• 4 space indented


• Multi-line


• Declarations in relevance (NOT alphabetical) order


• Indent vendor prefixed declarations so that their values are aligned


• Indent our rulesets to mirror the DOM


• Always include the final semi-colon in a ruleset

A brief example:

.widget 
 padding: 10px;
 border: 1px solid #BADA55;
 background-color: #C0FFEE;
 -webkit-border-radius: 4px;
 -moz-border-radius: 4px;
 border-radius: 4px;

 .widget-heading 
 font-size: 1.5rem;
 line-height: 1;
 font-weight: bold;
 color: #BADA55;
 margin-right: -10px;
 margin-left: -10px;
 padding: 0.25em;
 

Here we can see that .widget-heading must be a child of .widget as we have indented the.widget-heading ruleset one level deeper than .widget. This is useful information to developers that can now be gleaned just by a glance at the indentation of our rulesets.

We can also see that .widget-heading’s declarations are ordered by their relevance; .widget-heading must be a textual element so we begin with our text rules, followed by everything else.

One exception to our multi-line rule might be in cases of the following:

.t10 width: 10% 
.t20 width: 20% 
.t25 width: 25% /* 1/4 */
.t30 width: 30% 
.t33 width: 33.333% /* 1/3 */
.t40 width: 40% 
.t50 width: 50% /* 1/2 */
.t60 width: 60% 
.t66 width: 66.666% /* 2/3 */
.t70 width: 70% 
.t75 width: 75% /* 3/4*/
.t80 width: 80% 
.t90 width: 90% 

In this example (from inuit.css’s table grid system) it makes more sense to single-line our CSS.

Naming conventions

For the most part I simply use hyphen delimited classes (e.g. .foo-bar, not .foo_bar or .fooBar), however in certain circumstances I use BEM (Block, Element, Modifier) notation.

BEM is a methodology for naming and classifying CSS selectors in a way to make them a lot more strict, transparent and informative.

The naming convention follows this pattern:

.block 
.block__element 
.block--modifier 

• .block represents the higher level of an abstraction or component.


• .block__element represents a descendent of .block that helps form .block as a whole.


• .block--modifier represents a different state or version of .block.

An analogy of how BEM classes work might be:

.person 
.person--woman 
 .person__hand 
 .person__hand--left 
 .person__hand--right 

Here we can see that the basic object we’re describing is a person, and that a different type of person might be a woman. We can also see that people have hands; these are sub-parts of people, and there are different variations, like left and right.

We can now namespace our selectors based on their base objects and we can also communicate what job the selector does; is it a sub-component (__) or a variation (--)?

So, .page-wrapper is a standalone selector; it doesn’t form part of an abstraction or a component and as such it named correctly. .widget-heading, however, is related to a component; it is a child of the .widgetconstruct so we would rename this class .widget__heading.

BEM looks a little uglier, and is a lot more verbose, but it grants us a lot of power in that we can glean the functions and relationships of elements from their classes alone. Also, BEM syntax will typically compress (gzip) very well as compression favours/works well with repetition.

Regardless of whether you need to use BEM or not, always ensure classes are sensibly named; keep them as short as possible but as long as necessary. Ensure any objects or abstractions are very vaguely named (e.g..ui-list, .media) to allow for greater reuse. Extensions of objects should be much more explicitly named (e.g. .user-avatar-link). Don’t worry about the amount or length of classes; gzip will compress well written code incredibly well.

Classes in HTML

In a bid to make things easier to read, separate classes in your HTML with two (2) spaces, thus:

<div>

This increased whitespace should hopefully allow for easier spotting and reading of multiple classes.

JS hooks

Never use a CSS styling class as a JavaScript hook. Attaching JS behaviour to a styling class means that we can never have one without the other.

If you need to bind to some markup use a JS specific CSS class. This is simply a class namespaced with.js-, e.g. .js-toggle, .js-drag-and-drop. This means that we can attach both JS and CSS to classes in our markup but there will never be any troublesome overlap.

<th>
</th>

The above markup holds two classes; one to which you can attach some styling for sortable table columns and another which allows you to add the sorting functionality.

Internationalisation

Despite being a British developer—and spending all my life writing colour instead of color—I feel that, for the sake of consistency, it is better to always use US-English in CSS. CSS, as with most (if not all) other languages, is written in US-English, so to mix syntax like color: red; with classes like .colour-picker lacks consistency. I have previously suggested and advocated writing bilingual classes, for example:

.color-picker,
.colour-picker 

However, having recently worked on a very large Sass project where there were dozens of colour variables (e.g.$brand-color, $highlight-color etc.), maintaining two versions of each variable soon became tiresome. It also means twice as much work with things like find and replace.

In the interests of consistency, always name classes and variables in the locale of the language you are working with.

Comments

I use a docBlock-esque commenting style which I limit to 80 characters in length:

/**
 * This is a docBlock style comment
 *
 * This is a longer description of the comment, describing the code in more
 * detail. We limit these lines to a maximum of 80 characters in length.
 *
 * We can have markup in the comments, and are encouraged to do so:
 *
 <div>
 <p>Lorem</p>
 </div>
 *
 * We do not prefix lines of code with an asterisk as to do so would inhibit
 * copy and paste.
 */

You should document and comment our code as much as you possibly can, what may seem or feel transparent and self explanatory to you may not be to another dev. Write a chunk of code then write about it.

Comments on steroids

There are a number of more advanced techniques you can employ with regards comments, namely:

• Quasi-qualified selectors


• Tagging code


• Object/extension pointers

Quasi-qualified selectors

You should never qualify your selectors; that is to say, we should never write ul.nav  if you can just have.nav. Qualifying selectors decreases selector performance, inhibits the potential for reusing a class on a different type of element and it increases the selector’s specificity. These are all things that should be avoided at all costs.

However, sometimes it is useful to communicate to the next developer(s) where you intend a class to be used. Let’s take .product-page for example; this class sounds as though it would be used on a high-level container, perhaps the html or body element, but with .product-page alone it is impossible to tell.

By quasi-qualifying this selector (i.e. commenting out the leading type selector) we can communicate where we wish to have this class applied, thus:

/*html*/.product-page 

We can now see exactly where to apply this class but with none of the specificity or non-reusability drawbacks.

Other examples might be:

/*ol*/.breadcrumb 
/*p*/.intro 
/*ul*/.image-thumbs 

Here we can see where we intend each of these classes to be applied without actually ever impacting the specificity of the selectors.

Tagging code

If you write a new component then leave some tags pertaining to its function in a comment above it, for example:

/**
 * ^navigation ^lists
 */
.nav 

/**
 * ^grids ^lists ^tables
 */
.matrix 

These tags allow other developers to find snippets of code by searching for function; if a developer needs to work with lists they can run a find for ^lists and find the .nav and .matrix objects (and possibly more).

Object/extension pointers

When working in an object oriented manner you will often have two chunks of CSS (one being the skeleton (the object) and the other being the skin (the extension)) that are very closely related, but that live in very different places. In order to establish a concrete link between the object and its extension with use object/extension pointers. These are simply comments which work thus:

In your base stylesheet:

/**
 * Extend `.foo` in theme.css
 */
 .foo 

In your theme stylesheet:

/**
 * Extends `.foo` in base.css
 */
 .bar 

Here we have established a concrete relationship between two very separate pieces of code.

---

Writing CSS

The previous section dealt with how we structure and form our CSS; they were very quantifiable rules. The next section is a little more theoretical and deals with our attitude and approach.

Building new components

When building a new component write markup before CSS. This means you can visually see which CSS properties are naturally inherited and thus avoid reapplying redundant styles.

By writing markup first you can focus on data, content and semantics and then apply only the relevant classes and CSS afterwards.

OOCSS

I work in an OOCSS manner; I split components into structure (objects) and skin (extensions). As an analogy(note, not example) take the following:

.room 

.room--kitchen 
.room--bedroom 
.room--bathroom 

We have several types of room in a house, but they all share similar traits; they all have floors, ceilings, walls and doors. We can share this information in an abstracted .room  class. However we have specific types of room that are different from the others; a kitchen might have a tiled floor and a bedroom might have carpets, a bathroom might not have a window but a bedroom most likely will, each room likely has different coloured walls. OOCSS teaches us to abstract the shared styles out into a base object and then extend this information with more specific classes to add the unique treatment(s).

So, instead of building dozens of unique components, try and spot repeated design patterns across them all and abstract them out into reusable classes; build these skeletons as base ‘objects’ and then peg classes onto these to extend their styling for more unique circumstances.

If you have to build a new component split it into structure and skin; build the structure of the component using very generic classes so that we can reuse that construct and then use more specific classes to skin it up and add design treatments.

Layout

All components you build should be left totally free of widths; they should always remain fluid and their widths should be governed by a parent/grid system.

Heights should never be be applied to elements. Heights should only be applied to things which had dimensions before they entered the site (i.e. images and sprites). Never ever set heights on ps, uls,divs, anything. You can often achieve the desired effect with line-height which is far more flexible.

Grid systems should be thought of as shelves. They contain content but are not content in themselves. You put up your shelves then fill them with your stuff. By setting up our grids separately to our components you can move components around a lot more easily than if they had dimensions applied to them; this makes our front-ends a lot more adaptable and quick to work with.

You should never apply any styles to a grid item, they are for layout purposes only. Apply styling to contentinside a grid item. Never, under any circumstances, apply box-model properties to a grid item.

Sizing UIs

I use a combination of methods for sizing UIs. Percentages, pixels, ems, rems and nothing at all.

Grid systems should, ideally, be set in percentages. Because I use grid systems to govern widths of columns and pages, I can leave components totally free of any dimensions (as discussed above).

Font sizes I set in rems with a pixel fallback. This gives the accessibility benefits of ems with the confidence of pixels. Here is a handy Sass mixin to work out a rem and pixel fallback for you (assuming you set your base font size in a variable somewhere):

@mixin font-size($font-size) 
 font-size: $font-size +px;
 font-size: $font-size / $base-font-size +rem;

I only use pixels for items whose dimensions were defined before the came into the site. This includes things like images and sprites whose dimensions are inherently set absolutely in pixels.

Font sizing

I define a series of classes akin to a grid system for sizing fonts. These classes can be used to style type in a double stranded heading hierarchy. For a full explanation of how this works please refer to my article Pragmatic, practical font-sizing in CSS

Shorthand

Shorthand CSS needs to be used with caution.

It might be tempting to use declarations like background: red; but in doing so what you are actually saying is ‘I want no image to scroll, aligned top-left, repeating X and Y, and a background colour of red’. Nine times out of ten this won’t cause any issues but that one time it does is annoying enough to warrant not using such shorthand. Instead use background-color: red;.

Similarly, declarations like margin: 0; are nice and short, but be explicit. If you actually only really want to affect the margin on the bottom of an element then it is more appropriate to use margin-bottom: 0;.

Be explicit in which properties you set and take care to not inadvertently unset others with shorthand. E.g. if you only want to remove the bottom margin on an element then there is no sense in setting all margins to zero withmargin: 0;.

Shorthand is good, but easily misused.

IDs

A quick note on IDs in CSS before we dive into selectors in general.

NEVER use IDs in CSS.

They can be used in your markup for JS and fragment identifiers but use only classes for styling. You don’t want to see a single ID in any stylesheets!

Classes come with the benefit of being reusable (even if we don’t want to, we can) and they have a nice, low specificity. Specificity is one of the quickest ways to run into difficulties in projects and keeping it low at all times is imperative. An ID is 255 times more specific than a class, so never ever use them in CSS ever.

Selectors

Keep selectors short, efficient and portable.

Heavily location-based selectors are bad for a number of reasons. For example, take .sidebar h3 span . This selector is too location-based and thus we cannot move that span outside of a h3 outside of.sidebar and maintain styling.

Selectors which are too long also introduce performance issues; the more checks in a selector (e.g..sidebar h3 span has three checks, .content ul p a has four), the more work the browser has to do.

Make sure styles aren’t dependent on location where possible, and make sure selectors are nice and short.

Selectors as a whole should be kept short (e.g. one class deep) but the class names themselves should be as long as they need to be. A class of .user-avatar is far nicer than .usr-avt.

Remember: classes are neither semantic or insemantic; they are sensible or insensible! Stop stressing about ‘semantic’ class names and pick something sensible and futureproof.

Over-qualified selectors

As discussed above, qualified selectors are bad news.

An over-qualified selector is one like div.promo. You could probably get the same effect from just using.promo. Of course sometimes you will want to qualify a class with an element (e.g. if you have a generic.error class that needs to look different when applied to different elements (e.g. .error color: red; div.error padding: 14px; )), but generally avoid it where possible.

Another example of an over-qualified selector might be ul.nav li a . As above, we can instantly drop theul and because we know .nav is a list, we therefore know that any a must be in an li, so we can getul.nav li a  down to just .nav a .

Selector performance

Whilst it is true that browsers will only ever keep getting faster at rendering CSS, efficiency is something you could do to keep an eye on. Short, unnested selectors, not using the universal (* ) selector as the key selector, and avoiding more complex CSS3 selectors should help circumvent these problems.

CSS selector intent

Instead of using selectors to drill down the DOM to an element, it is often best to put a class on the element you explicitly want to style. Let’s take a specific example with a selector like .header ul …

Let’s imagine that ul is indeed the main navigation for our website. It lives in the header as you might expect and is currently the only ul in there; .header ul  will work, but it’s not ideal or advisable. It’s not very future proof and certainly not explicit enough. As soon as we add another ul to that header it will adopt the styling of our main nav and the the chances are it won’t want to. This means we either have to refactor a lot of code or undo a lot of styling on subsequent uls in that .header to remove the effects of the far reaching selector.

Your selector’s intent must match that of your reason for styling something; ask yourself ‘am I selecting this because it’s a ul inside of .header or because it is my site’s main nav?’. The answer to this will determine your selector.

Make sure your key selector is never an element/type selector or object/abstraction class. You never really want to see selectors like .sidebar ul  or .footer .media  in our theme stylesheets.

Be explicit; target the element you want to affect, not its parent. Never assume that markup won’t change. Write selectors that target what you want, not what happens to be there already.

For a full write up please see my article Shoot to kill; CSS selector intent

!important

It is okay to use !important on helper classes only. To add !important preemptively is fine, e.g..error color: red !important , as you know you will always want this rule to take precedence.

Using !important reactively, e.g. to get yourself out of nasty specificity situations, is not advised. Rework your CSS and try to combat these issues by refactoring your selectors. Keeping your selectors short and avoiding IDs will help out here massively.

Magic numbers and absolutes

A magic number is a number which is used because ‘it just works’. These are bad because they rarely work for any real reason and are not usually very futureproof or flexible/forgiving. They tend to fix symptoms and not problems.

For example, using .dropdown-nav li:hover ul top: 37px;  to move a dropdown to the bottom of the nav on hover is bad, as 37px is a magic number. 37px only works here because in this particular scenario the.dropdown-nav happens to be 37px tall.

Instead you should use .dropdown-nav li:hover ul top: 100%;  which means no matter how tall the.dropdown-nav gets, the dropdown will always sit 100% from the top.

Every time you hard code a number think twice; if you can avoid it by using keywords or ‘aliases’ (i.e.top: 100% to mean ‘all the way from the top’) or—even better—no measurements at all then you probably should.

Every hard-coded measurement you set is a commitment you might not necessarily want to keep.

Conditional stylesheets

IE stylesheets can, by and large, be totally avoided. The only time an IE stylesheet may be required is to circumvent blatant lack of support (e.g. PNG fixes).

As a general rule, all layout and box-model rules can and will work without an IE stylesheet if you refactor and rework your CSS. This means you never want to see<!--[if IE 7]> element margin-left: -9px; < ![endif]--> or other such CSS that is clearly using arbitrary styling to just ‘make stuff work’.

Debugging

If you run into a CSS problem take code away before you start adding more in a bid to fix it. The problem exists in CSS that is already written, more CSS isn’t the right answer!

Delete chunks of markup and CSS until your problem goes away, then you can determine which part of the code the problem lies in.

It can be tempting to put an overflow: hidden; on something to hide the effects of a layout quirk, but overflow was probably never the problem; fix the problem, not its symptoms.

Preprocessors

Sass is my preprocessor of choice. Use it wisely. Use Sass to make your CSS more powerful but avoid nesting like the plague! Nest only when it would actually be necessary in vanilla CSS, e.g.

.header 
.header .site-nav 
.header .site-nav li 
.header .site-nav li a 

Would be wholly unnecessary in normal CSS, so the following would be bad Sass:

.header 
 .site-nav 
 li 
 a 
 
 

If you were to Sass this up you’d write it as:

.header 
.site-nav 
 li 
 a 

CSS guidelines A developer Must Know