Compared to <link>s, @import is slower, adds extra page requests, and can cause other unforeseen problems. Avoid them and instead opt for an alternate approach
Compile your CSS with a preprocessor like Sass or Less into a single file
Place media queries as close to their relevant rule sets whenever possible.
Limit shorthand declaration usage to instances where you must explicitly set all available values.
Avoid unnecessary nesting. Just because you can nest, doesn't mean you always should. Consider nesting only if you must scope styles to a parent and if there are multiple elements to be nested.
For a complete list of properties and their order, please see
The :required attribute works on radio buttons. If you put required on one radio button (or all), that particular group of radio buttons will be required. On checkboxes, makes each individual checkbox required (to be checked).