[css21][css-inline] height of inline box with line-height:normal and primary font not used

[frivoal]

This is part of the #1796 series. Read that issue for context

Test source
Ref source (mismatch)
Live test
Live ref (mismatch)

As seen in the test above, in the general case, if a inline-level box contains only characters from fallback fonts, all browsers agree to size the height of a line-height:auto inline-level box using the metrics of the fonts actually used plus the metrics of the primary font, even if it is not used. I.E. they take the strut into account.

However, see #1798 for a possible exception.

[frivoal]

This means that the following sentence in CSS21 should be extended:

If the inline box contains no glyphs at all, it is considered to contain a strut (an invisible glyph of zero width) with the A and D of the element's first available font.

to

If the inline box contains no glyphs at all, or if it contains only glyphs from fallback fonts, it is considered to contain a strut (an invisible glyph of zero width) with the A and D of the element's first available font.