Why Can't I Combine CSS Vendor-Specific Pseudo-Elements/Classes?

Why CSS Vendor-Specific Pseudo-Elements/Classes Can't Be Combined

It is a common practice in CSS to use vendor-specific pseudo-classes and pseudo-elements to achieve cross-browser compatibility for styling placeholder text in input fields. However, combining these vendor-specific selectors into a single rule set, as one might with regular CSS selectors, is not possible due to limitations in CSS validation.

CSS2.1 defines the selector to include everything up to the opening curly brace of a declaration block. If a user agent encounters a selector that it cannot parse, it must disregard both the selector and the corresponding declaration block.

In the context of vendor-specific prefixes, this limitation arises because browsers often do not understand prefixes from other vendors. When a rule contains a vendor-specific prefix in a pseudo-class or pseudo-element selector, the browser may not recognize it and thus drop the entire rule.

For example, if a comma-separated rule set includes a selector with the :-moz-placeholder pseudo-class, browsers that do not support Mozilla-specific prefixes (such as Chrome or Safari) will ignore the entire rule because they cannot parse that particular selector. This renders the combination method ineffective.

It is important to note that some browsers, such as WebKit, may partially deviate from this rule and allow unrecognized pseudo-elements in certain contexts. However, combining vendor-specific selectors remains inconsistent across browsers and can lead to unpredictable styling behavior.

Consequently, to avoid this limitation and ensure consistent placeholder styling, it is necessary to continue using separate vendor-specific selectors for different browsers.

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