How to deal with absolute dimensions

[gagern]

In KaTeX, all dimensions are given in em, which is relative to the current font size. However, there are some dimensions in the TeX world which are given in pt or some similar absolute value. Currently, expressing such dimensions in the DOM is either pretty complicated (manually resetting the size and style up front) or not accurate (if the current font is not the 10pt default).

The prime example which comes to my mind here is the \nulldelimiterspace=1.2pt discussed in #135 (diff) and #249 (comment). If things are to look the way they do in TeX, then we have to make that box always 1.2/ptPerEm times the size of the unsized text style font size.

But how? I can think of several alternatives.

1. Add classes to reset to textstyle and size5, so that the size is always relative to the default font size. This is what I did in @7dc8b68.
2. Express the spacing as kerning (using makeKern) and then provide some function which takes the current options and the size in pt and does all the neccessary computation. Added support for \cfrac #135 is using makeKern, but so far the resulting size is still relative to the current size and style.
3. Deliberately deviate from LaTeX and declare that all dimensions should scale with the font size.

[kevinbarabash]

This has been addressed in #732.