alt131 - 1:17 pm on Dec 22, 2011 (gmt 0)
Not quite: Older ie would not allow fonts specified in px to scale, and keywords were not consistent cross-browser or -platform. That meant ems or % for finer control. Some lengths could not be specified in %, or some browsers weren't that accurate, especially to decimal places, so that left ems. Assuming 16px as the default size, which was frequently too large, 62.5% as the base meant 1em=10px, 1.2em=12px, etc.*** See Clagnut's How to size text using ems [clagnut.com] and the ALA article. So the %/em combination was the least complicated way to achieve consistent font-sizes while allowing some degree of control as required for design.
Why mix and match? Introduces unnecessary complications.
cheaperholidays, not quite sure what you mean by that, but 62.5%/10px was always considered too small for readability by many of the advocates of this technique, depending on font and colour 12px may be too small as well - and 100% too large. For that reason another common setting was 76%, which broke the firstname.lastname@example.org%=10px relationship, but created a workable size to begin with.
the 62.5% thing should have worked
I don't think the issue is whether a font is a particular size or is specified in a certain way. For me the issue is whether it is readable.
So, given all the possible user, browser and platform variations, I think this is coder choice - use what will allow you to deliver the most readable sizes to the user as efficiently as possible. w3org has a tip sheet on techniques in Care With Font Size [w3.org]. That said, I'm old enough to still be wary of keywords. %+ems works for me ;)
***which made the maths easy for coders.