Support for ems in leading field of the type panel
When setting the leading, we have the option of "Auto" or setting a specific value in a static unit like px or pt.
It would be great if a dynamic unit like ems (as implemented in CSS) were available to make leading, and I suppose other type attributes, scalable relative to the font size.
For example, if I have a block of text set to 12pt and 1.5em, the leading would be 1.5x the point size, and therefore 18pt. If I were to alter the font size to 20px, the leading would still be 1.5em and therefore would scale automatically to 30px.
This could also then output to the CSS Properties panel as ems.

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Douglas Brandt commented
Agreed whole-heartedly...
If not in terms of ems (which may be too CSS-specific for some traditional designers), then at least expressed as a percentage of the font size in use..
I know this is the same thing, but non-coders might understand percentage rather than em. ;-)