size-adjust should scale ascent and descent metrics in 'line-height: normal'.
In a text node with multiple fonts, size-adjust should only scale characters in the modified font. In the test case below, digits should be scaled, but not letters.
In nested elements, size-adjust should be applied to each text segment independently. Both elements below should be scaled to 50% of their original sizes, respectively.
The computed 'font-size' value (i.e., 'em') should not be affected. But 'ex' should be scaled since it's measured from the actual glyph.