htmlpurifier/docs/ref-css-length.txt

CSS Length Reference
  To bound, or not to bound, that is the question

It's quite a reasonable request, really, and it's already been implemented
for HTML.  That is, length bounding.  It makes little sense to let users
define text blocks that have a font-size of 63,360 inches (that's a mile,
by the way) or a width of forty-fold the parent container.

But it's a little more complicated then that. There are multiple units
one can use, and we have to a little unit conversion to get things working.
Here's what we have:

Absolute:
    1 in ~= 2.54 cm
    1 cm = 10 mm
    1 pt = 1/72 in
    1 pc = 12 pt

Relative:
    1 em ~= 10.0667 px
    1 ex ~= 0.5 em, though Mozilla Firefox says 1 ex = 6px
    1 px ~= 1 pt

Watch out: font-sizes can also be nested to get successively larger
(although I do not relish having to keep track of context font-sizes,
this may be necessary, especially for some of the more advanced features
for preventing things like white on white).