I'm curious to know where in the CSS specifications it is specified how multiple properties with the same name should be handled

While debugging an unrelated issue, I want to make sure I understand how a browser is supposed to handle a declaration like

background-image: url(image0.jpg);
background-image: image-set(url(image1.jpg) 1x, url(image2.jpg) 2x);

I tried looking in the CSS specs for clarification but couldn't find anything specific.

Based on my testing, it seems that browsers will prioritize the first valid declaration they come across. However, I am seeking confirmation from an authoritative source - preferably a specification document. Please note that I'm not just asking about the expected behavior; I'm specifically interested in citing a credible source.

css

Answer №1

In exploring my own question, I came to the realization that it was crucial to investigate the answers to two sub-questions: "which out of two declarations wins" and "what should it do about invalid declarations."

Regarding the former inquiry, a source at https://www.w3.org/TR/css-cascade-3/#cascade-order states:

The last declaration in document order wins.

This implies that when faced with two property-value declarations sharing the same property, the final one will prevail.

As for the latter concern, another source at https://www.w3.org/TR/css-cascade-3/#w3c-partial conveys:

User agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

In essence, this means that if a browser, for example, does not recognize image-set in the scenario presented in the question, it will disregard the entire declaration containing it.

Answer №2

According to the guidelines:

The final computation of a CSS property for an element involves several steps:

  1. Initially, all declared values applied to an element are gathered, considering each property on the element. There can be none or multiple declared values for the element.
  2. Cascading determines the cascaded value. Each property per element has at most one cascaded value.

Additionally,

Every property declaration applied contributes a declared value associated with that property and element. More information can be found in Filtering Declarations. reference

Referring to Filtering Declarations:

To determine declared values, implementations must first identify all declarations applicable to each element. A declaration is considered applicable if:

  • It is part of a style sheet currently affecting the document.
  • It is not subject to a conditional rule [CSS3-CONDITIONAL] with a false condition.
  • It corresponds to a style rule whose selector matches the element. [SELECT]
  • It is syntactically correct: the property in the declaration is recognized, and the value follows the syntax of that property.

The resulting list of declared values for each property on every element forms the basis for prioritization in the cascade, as explained in the next section.

Regarding the cascade process:

The cascade organizes an unordered collection of declared values for a specific property on an element, sequences them based on precedence defined below, and selects a single cascaded value.

If you are familiar with how cascade works (specificity, order, etc.), you understand that the last valid property will take precedence. If it's invalid, the first valid property is considered. Otherwise, the default value applies.

Answer №3

Let's dive into the reasoning behind this.
The rationale for this is akin to variable mutation in various programming languages. However, unlike other programs, CSS is designed to be resilient and will not reassign invalid properties.

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