[fxtf-drafts] [css-masking-1] "Computed value" of property `mask-position` seems incorrect/inconsistent (#605)

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== [css-masking-1] "Computed value" of property `mask-position` seems incorrect/inconsistent ==
The _Computed value_ for `mask-position` seems incorrect or at least inconsistent with regards to the definition of `background-position`.

Currently the definition states:

> Consisting of: two keywords representing the origin and two offsets from that origin, each given as an absolute length (if given a \<length\>), otherwise as a percentage.

This reads to me as e.g. `left top 10% 5px` or `left bottom 5px 10px`. Or possibly as `left 10% top 5px`. Observed behavior in the major browser engines seems to be an implied left-top corner and only explicit length-percentages, i.e. `10% 5px`. This matches the way I read the corresponding description of `background-position` (emphasis mine):

> list, each item a **pair of offsets (horizontal and vertical) from the top left origin each given as a computed \<length-percentage\> value**

So I think the definition for `mask-position` should be aligned with the one for `background-position`.

Note that WebKit and Blink show some inconsistencies around this, but those are the same inconsistencies that can be seen on [background-position-computed.html](https://wpt.live/css/css-backgrounds/parsing/background-position-computed.html)

Please view or discuss this issue at https://github.com/w3c/fxtf-drafts/issues/605 using your GitHub account


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Received on Sunday, 3 August 2025 19:15:43 UTC