Why Can't We Combine Vendor-Specific Placeholder Selectors in a Single CSS Rule?

Unveiling the Enigma of Combining Vendor-Specific Selectors

Within the realm of CSS, the conundrum of styling placeholder text across multiple browsers is encountered. To achieve optimal cross-browser support, a combination of vendor-specific pseudo-classes and pseudo-elements is employed. However, there lies a perplexing quandary: why is it not feasible to merge these into a single rule set as is customary in other CSS scenarios?

Delving into the depths of CSS2.1 specifications unravels the answer. It mandates that a selector is defined as everything leading up to the first left curly brace ({). When a user agent encounters a selector it cannot decipher, it must disregard both the selector and the ensuing declaration block.

The implication of this is that browsers with certain prefixes may not comprehend the selectors with other vendor-specific prefixes, leading to the exclusion of rules mentioning these unrecognized prefixes in selectors. Consequently, attempts to combine these rules into a single, comma-separated rule set, like this:

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

will prove futile. The user agent will only apply the styles to the recognized selector, resulting in inconsistent styling across browsers.

This intricate rule serves a purpose. It ensures that browsers adhere to recognized CSS syntax and only implement rules they fully understand. By enforcing this, any potential inconsistencies or rendering errors due to vendor-specific deviations are avoided.

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