Unless dealing with CSS validator bugs or requiring proprietary syntax, use valid CSS code.

Use tools such as the W3C CSS validator to test.

Using valid CSS is a measurable baseline quality attribute that allows to spot CSS code that may not have any effect and can be removed, and that ensures proper CSS usage.

Instead of presentational or cryptic names, always use ID and class names that reflect the purpose of the element in question, or that are otherwise generic.

Names that are specific and reflect the purpose of the element should be preferred as these are most understandable and the least likely to change.

Generic names are simply a fallback for elements that have no particular or no meaning different from their siblings. They are typically needed as “helpers.”

Using functional or generic names reduces the probability of unnecessary document or template changes.

Try to convey what an ID or class is about while being as brief as possible.

Using ID and class names this way contributes to acceptable levels of understandability and code efficiency.

Unless necessary (for example with helper classes), do not use element names in conjunction with IDs or classes.

Avoiding unnecessary ancestor selectors is useful for performance reasons.

CSS offers a variety of shorthand properties (like font) that should be used whenever possible, even in cases where only one value is explicitly set.

Using shorthand properties is useful for code efficiency and understandability.

Do not use units after 0 values unless they are required.

Do not use put 0s in front of values or lengths between -1 and 1.

For color values that permit it, 3 character hexadecimal notation is shorter and more succinct.

In large projects as well as for code that gets embedded in other projects or on external sites use prefixes (as namespaces) for ID and class names. Use short, unique identifiers followed by a dash.

Using namespaces helps preventing naming conflicts and can make maintenance easier, for example in search and replace operations.

Do not concatenate words and abbreviations in selectors by any characters (including none at all) other than hyphens, in order to improve understanding and scannability.

It’s tempting to address styling differences over user agent detection or special CSS filters, workarounds, and hacks. Both approaches should be considered last resort in order to achieve and maintain an efficient and manageable code base. Put another way, giving detection and hacks a free pass will hurt projects in the long run as projects tend to take the way of least resistance. That is, allowing and making it easy to use detection and hacks means using detection and hacks more frequently—and more frequently is too frequently.

Put declarations in alphabetical order in order to achieve consistent code in a way that is easy to remember and maintain.

Ignore vendor-specific prefixes for sorting purposes. However, multiple vendor-specific prefixes for a certain CSS property should be kept sorted (e.g. -moz prefix comes before -webkit).

Indent all block content, that is rules within rules as well as declarations, so to reflect hierarchy and improve understanding.

End every declaration with a semicolon for consistency and extensibility reasons.

Always use a single space between property and value (but no space between property and colon) for consistency reasons.

Always start a new line for each selector and declaration.

Always put a line between rules.

Use single ('') rather than double ("") quotation marks for attribute selectors or property values. Do not use quotation marks in URI values (url()).

Exception: If you do need to use the @charset rule, use double quotation marks—single quotation marks are not permitted.

If possible, group style sheet sections together by using comments. Separate sections with new lines.