Why Can't I Combine Vendor-Specific Pseudo-elements and Classes in a Single CSS Rule?

Mixing Vendor-Specific Pseudo-Elements and Classes in CSS

In CSS, we often use vendor-specific pseudo-elements and classes to enhance browser compatibility. However, it's not possible to combine these into one rule set despite sharing similar properties.

Why Not?

CSS2.1 dictates that a selector must be valid before the user agent applies the declaration block. Prefixing vendor names in selectors introduces unrecognized characters for some browsers. Hence, user agents must drop these rules to maintain validity.

Specifically:

• Vendors' browsers do not understand each other's prefixes.
• A comma-separated selector containing vendor prefixes will have at least one unrecognizable selector for each browser, causing the entire rule to be ignored.

Impact on Styling

When styling placeholder text, for example, this limitation forces us to write multiple redundant rules, as each vendor requires a separate prefix:

input:-moz-placeholder {
  font-style: italic;
  text-align: right;
}
input::-moz-placeholder {
  font-style: italic;
  text-align: right;
}
input:-ms-input-placeholder {
  font-style: italic;
  text-align: right;
}
input::-webkit-input-placeholder {
  font-style: italic;
  text-align: right;
}

Conclusion

Understanding this limitation allows us to create valid and consistent CSS styles. While vendor prefixes are sometimes necessary for browser compatibility, mixing them in a single rule set remains infeasible due to browser-specific parsing rules.

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