CSSLint

Introduction

CSSLint is a tool to help point out problems with your CSS code. It does basic syntax checking as well as applying a set of rules to the code that look for problematic patterns or signs of inefficiency. The rules are all pluggable, so you can easily write your own or omit ones you don't want.

The CSSLint Rules

Parsing errors should be fixed

By default, CSSLint shows any parsing errors. Parsing errors usually mean you mistyped a character and may cause the browser to drop your rule or a property. Parsing errors are presented as errors by CSSLint, the most important issues to fix.

Don't use adjoining classes

Adjoining classes look like .foo.bar. While technically allowed in CSS, these aren't handled properly by Internet Explorer 6 and earlier.

Remove empty rules

Any rule that doesn't contain any properties, such as:

.foo {
}

A lot of times, empty rules appear as a result of refactoring without further cleanup. Eliminating empty rules results in smaller file sizes and less style information for the browser to deal with.

Use correct properties for a display

Even though you can define any group of properties together in a CSS rule, some of them will be ignored due to the display of the element. This leads to extra cruft in the CSS file. The list of properties that CSSLint checks for are:

display: inline should not use width, height, margin (and all variants), padding (and all variants), or float.
display: inline-block should not use float.
display: block should not use vertical-align.
display: table-* should not use margin (and all variants) or float.

Removed the ignored or problematic properties decreases file size and improves performance.

Avoid using to many !important declarations

Using !important overides any cascaded rule and may lead to specificity war. CSSLint checks if you've used !important, and if so, displays a warning. If there's at least 10 !important declaration in your code CSSLint displays an error.

Don't use too many floats

Using float for layout isn't a great idea, but sometimes you have to. CSSLint simply checks to see if you've used float more than 10 times, and if so, displays a warning. Using this many floats usually means you need some sort of abstraction to achieve the layout.

Don't use too many web fonts

Web fonts are growing in popularity and use of @font-face is on the rise. However, using web fonts comes with performance implications as font files can be quite large and some browsers block rendering while downloading them. For this reason, CSSLint will warn you when there are more than five web fonts in a style sheet.

Don't use too may font-size declarations

A site is typically made up of a finite number of font treatments, including font size. If you have 10 or more font sizes specified, you probably want to refactor into a standard set of font size classes that can be used in markup.

Don't use IDs in selectors

IDs shouldn't be used in selectors because these rules are too tightly coupled with the HTML and have no possibility of reuse. It's much preferred to use classes in selectors and then apply a class to an element in the page.

Don't qualify headings

Heading elements (h1-h6) should be defined as top-level styles and not scoped to particular areas of the page. For example, this is an example of an overqualified heading:

.foo h1 {
    font-size: 110%;
}

Heading elements should have a consistent appearance across a site.

Heading styles should only be defined once

Heading elements (h1-h6) should have exactly one rule on a site. CSSLint warns if it finds more than one.

Be careful using width: 100%

Using width: 100% on an element whose parent element has padding will result in the child stretching outside of the parent's bounding box. It's generally not a good idea to use width: 100%. Instead, use width: auto or display: block.

Zero values don't need units

An easy way to save bytes in CSS is not include units when a value is 0. For instance, 0px and 0 are the exact same measurement, so leave off the units and save!

Vendor prefixed properties should also have the standard

When using vendor-prefixed properties such as -moz-border-radius, make sure to also include the standard property. The standard property should preferably come after the vendor-prefixed one, such as:

.foo {
    -moz-border-radius: 5px;
    border-radius: 5px;
}

CSS gradients require all browser prefixes

Right now, there is no standard CSS gradient implementation, which means using CSS gradients in a cross-browser way requires using many different vendor-prefixed versions. CSSLint warns when a rule with a CSS gradient doesn't have gradients for all supporting browsers.

Avoid selectors that look like regular expressions

CSS3 adds complex attribute selectors such as ~= that are slow. When using attribute selectors, don't use the complex equality operators to avoid performance penalties.

Beware of broken box models

Borders and padding add space outside of an element's content. Setting width or height along with borders and padding is usually a mistake because you won't get the visual result you're looking for. CSSLint warns when a rule uses width or height in addition to padding and/or border.

timlo / csslint