[css-values][cssom] Which unit to serialize <resolution> with?

[tabatkins]

CSSOM defines that <resolution> values must be serialized with the dpcm unit. I don't think anyone does that, and it seems like the least useful of all the resolution units.

Most useful is probably dppx, but dpi is also common. Which one should we use?

[astearns]

The working group resolved to use dppx https://lists.w3.org/Archives/Public/www-style/2016Jun/0118.html. Removing the agenda tag