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General CSS notes, advice and guidelines

Follow me on Twitter and tweet these guidelines.

CSS documents

We maintain a table of contents at the top of each CSS file which maps to sections in the document. Each section is prefixed with a $ symbol which means that doing a find for $[section name] will only yield results that are sections.

Syntax and formatting

We use multi-line CSS to help with version control (diffing single line CSS is a nightmare) and we order CSS declarations by relevance, not alphabetically.

We use hyphen delimited, lowercase selectors: .thisIsBad{}, .this_is_also_bad{} but .this-is-correct{}.

Always use a trailing semi-colon on the last declaration in a ruleset to avoid any potential confusion and syntax errors over the life of the document.

For an example of our preferred CSS file formatting and structure please see…/style.css



Comment as much as you can as often as you can. Where it might be useful, include a commented out piece of markup which can help put the current CSS into context.

Be verbose, go wild, CSS will be minified before it hits live servers.


For each level of markup nesting, try and indent your CSS to match. For example:

    .nav li{}
        .nav a{}
    .promo p{}

Also write vendor prefixed CSS so that colons all line up, thus:


This means that we can quickly scan down and see that they are all set to 4px, but more importantly—if our text editor supports it—we can type in columns to change all the values at once.

Building 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.


When building components try and keep a DRY, OO frame of mind.

Instead of building dozens of unique components, try and spot repeated design patterns abstract them; 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.



All components should be left totally free of widths; your components should always remain fluid and their widths should be governed by a 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 normally achieve the desired effect with line-heights which are 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.

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


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


Font sizing

We use rems (with a pixel fallback for older browsers only). We do not want to define any font sizes in pixels as standard. We define line heights unitlessly everywhere unless we are trying to align text to known heights.

We want to avoid defining font sizes over and over; to achieve this we have a predefined scale of font sizes tethered to classes. We can recycle these rather than having to declare styles over and over.

Before writing another font-size declaration, see if a class for it already exists.



It might be tempting to use declarations like background:red; but in doing so what we are actually saying is ‘I want no image to scroll, aligned top left and 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’re actually only really wanting 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 blitzing all margins with margin:0;.

Shorthand is good, but easily misused.


Keep selectors 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.

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

An over-qualified selector is one like We could probably get the same effect from just using .promo. Of course sometimes we 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 the ul and because we know .nav is a list, we therefore know that any a must be in an li, so we can get ul.nav li a{} down to just .nav a{}.


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


Be explicit, don’t make assumptions

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.

Imagine you have a promotional banner with a class of .promo and in there there is some text and call-to-action link. If there is just one a in the whole of .promo then it may be tempting to style that call-to-action via .promo a{}.

The problem here should be obvious in that as soon as you add a simple text link (or any other link for that matter) to the .promo container it will inherit the call-to-action styling, whether you want it to or not. In this case you would be best to explicitly add a class (e.g. .cta) to the link you want to affect.

Be explicit; target the element you want to affect, not its parent. Never assume that markup won’t change.

IDs and classes

Do not use IDs in CSS at all. They can be used in your markup for JS and fragment-identifiers but use only classes for styling. We don’t want to see a single ID in this (or any other) stylesheet.

Classes come with the benefit of being reusable (even if we don’t want to, we can) and they have a nice, low specificity.



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 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 we 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 we 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’.


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.


By following the above advice you should typically find the need for a preprocessor decreases dramatically. If you still wish to use a preprocessor then by all means do so, but only as en extension of the above, not an alternative.

For example, preprocessors’ nesting abilities often lead to overly specific and location dependent selectors. Let’s use our . nav a{} example again:


Will compile to:

.nav {}
.nav li {}
.nav li a {}

Whilst this is a very timid example, it does help illustrate how a lot of preprocessors’ built in ‘helpful’ aspects actually go against our ideals; .nav li a{} could (and should) just be .nav a{}.

Also, with mixins and the like, preprocessors teach you how to recognise abstractions—which is great—but not necessarily how to use them properly; there’s no point writing an abstracted mixin when you proceed to repeat it a dozen times in a stylesheet.

Be sure to know the ins-and-outs of excellent vanilla CSS and where a preprocessor can aid that, not hinder or undo it. Learn the downsides of preprocessors inside-out and then fuse the best aspects of the two with the bad bits of neither.