font: .81em/150% Arial; why is it 0.81?

Question

I saw this font declaration on a web page and I don't understand what it does. I googled, but no results. Any idea?

font: .81em/150% Arial, Helvetica, sans-serif;

I know what the Arial, Helvetic, sans-serif part of it does; I'm wondering what the .81em does.

why is it 0.81? why not it is 0.8 or 0.82 or other value?

Answer 1

This is a short to define font-size AND line-height at once.

0.81 is probably random, just a try which fits

Answer 2

Probably someone wanted to set the font-size to 13px and used some conversion tool to em, and then found out, that e.g. this table has .813em for 13px, and then did round down.