CSS: Don't target multiple vendor-prefixed pseudo-elements in a single rule
Some pseudo-elements need to be addressed with vendor prefixes. E.g. ::selection is not supported by Firefox, you need to use ::-moz-selection instead.
What you cannot do is to define a single CSS rule to address both the standard and vendor-prefixed form:
Copy::selection, ::-moz-selection { background-color: red; }
This rule will be ignored by all browsers. The reason is that if a browser does not know all pseudo elements in a selector, it ignores the entire rule. This is because the CSS spec says so.
So you need to copy & paste the rule instead:
Copy::selection { background-color: red; } ::-moz-selection { background-color: red; }
If you're using a CSS preprocessor like Sass, you can write a mixin that does the copying and pasting for you:
Copy=selection ::-moz-selection @content ::selection @content
Then use it like this in your .sass file:
Copy+selection background-color: red
You will still have the rule twice in your CSS, but your .sass will be DRY (and the CSS will compress well before delivery).
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