> I understand why you may see this as a bug, but for me it's a feature.
> Personal experience tells me that mixing units -- even if they belong
> to the same system, as cm and mm -- is asking for trouble (never mind
> mixing units from metric and imperial -- ask NASA).

Well you're not in a NASA case since all the data is tagged correctly.

> But anyway, if you really think unit conversion is a must-have feature,
> I can add it to the next release (it's actually simple to implement).
> However, personally I remain sceptical about its real-world usefulness.

For the web I anyway use more relative lengths than absolute ones. I
don't know if it's a must have feature however I can see that being
annoying if you have to do an absolute layout (e.g. for print).

The problem is that different kind of objects naturally use different
kind of units.
A font size or line height for example is usually expressed in pts and
a standard paper or photographic print size will be in cms (or
inches).

Having to use a calculator to add them just feels wrong and makes the
resulting stylesheet less readable : if I see 210mm x 297mm I
recognize quite easily A4, however if I see 595.275591pt x
841.889764pt, well, no. Conversely a value in pt for a line height or
font size will give me a better idea of the typographical result than
a value in mm.

Best,

Daniel

P.S.
I know about metric typographic units [1] but somehow they didn't
catch and most people still think in pts in typography.

[1] http://www.cl.cam.ac.uk/~mgk25/metric-typo/