[css-viewport][css-fonts] Effect of the zoom property on font metrics as applied to length units #14260

Consider the following scenario:

.test {
    font-size: 10px;
    zoom: 2;
    tab-size: 1ex;
}

In this scenario, the computed value for tab-size depends on how / when font metrics get computed.

Option 1 would be for the engine to perform font resolution using the computed value of font-size 10px, and then at use time, scaling all metrics by the zoom.

Option 2 (and what at least WebKit is currently doing) would be for the engine to perform font resolution using the used value of the font-size 20px and using it directly.

With this option, the naive computed value for tab-size would be way too big, as it would be using the x-height for a 20px font-size. What WebKit does to compensate is that it unscales the x-height by the zoom to bring it back into proportion. This is problematic as, unless x-height scales exactly linearly with font-size, the value can be distinct from the equivalent example without zoom:

.test {
    font-size: 10px;
    tab-size: 1ex;
}

Option 3 would be perform font resolution for both the computed and used font sizes, stashing the metrics for the unzoomed value away for use by units.

--

So, the questions are:

• Does CSS Fonts define which font-size to use for font resolution when zoom is in play? (I wasn't able to find anything, but most things don't specify custom behavior with zoom, so this isn't unexpected).
• If not, is there consensus on which option above is correct?

cc @emilio @svgeesus