[css-backgrounds-3] Computed values of background-repeat don't follow reality

[nox]

The spec says:

Computed value: A list, each item consisting of: two keywords, one per dimension

But Safari, Chrome and Firefox do otherwise and just serialise computed values as specified.

<div style="background-repeat: space"></div>
<pre>
<script>
    var div = document.getElementsByTagName("div")[0];
    document.write(div.style.backgroundRepeat, "<br>");
    document.write(getComputedStyle(div).backgroundRepeat);
</script>

[nox]

Note also that if one runs that sample with space space, all three user agents I mentioned serialise the computed value as space, and Chrome is the only one to shorten the specified value from space space to space.

[tabatkins]

"Computed value" and "serialization" are distinct, unrelated concepts. Computed value is a CSS-internal concept used by the specs.

The "shortest equivalent" rule is usually in effect for serializing computed values, which is why everyone serializes space space to space for computed values. Chrome is wrong to shorten the specified value; that should match what was input.

[nox]

From what should I infer that when reading "A list, each item consisting of: two keywords, one per dimension" though? I don't understand why everyone should serialise to space when it says "two keywords" and nothing else, for computed values.

[tabatkins]

Because, as I said, "computed value" and "serialization" have nothing directly to do with each other. The serialization of the value is not specified in the propdef table; it's implicitly specified in CSSOM as the shortest equivalent serialization that'll produce the same value.

The computed value is a pair of values. We can operate on that in spec text, asking what the horizontal background-repeat is, versus the vertical background-repeat. This avoids us having to go thru contortions in spec text to reproduce the effects of parsing, like "do X if the vertical repeat is Y, or if it's unspecified and the horizontal repeat is Y".

[nox]

I see. Closing.