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Absolute Length Values
Notation Meaning
in inches
cm centimetres
mm millimetres
pt Printing industry point; originally there was 72pt to a French inch which was larger than an English inch! Postscript and CSS define 1pt=1/72in
pc picas; 1pc=12pt

These should all be the same size:

0.25in para, 0.635cm text, 6.35mm text, 18pt text, 1.5pc text in side

0.25in para, 0.635cm text, 6.35mm text, 18pt text, 1.5pc text in side

0.25in para, 0.635cm text, 6.35mm text, 18pt text, 1.5pc text in side

Firefox, Chrome and Safari good, older Opera and IE versions have inches varying by the font!

All three have the text size the same for other absolute values in the para