http://www.w3.org/TR/REC-CSS2/syndata.html#length-units
If you have a problem reading text on the screen, I suggest you use something like Clearly extension that reformats pages with a much more reading-friendly style.
You don't have to sign up to use this extension : https://evernote.com/clearly/
You can also try to print the page into a PDF and read it like that.
> If the pixel density of the output device is very different from that of a typical computer display, the user agent should rescale pixel values.
So actually 12px font should look about the same on all output devices, regardless of their physical DPI. But I agree that 12px is rather small for copy text.
I guess a conforming browser should randomly sample computer displays before deciding how big 1px is.
Serious version: I don't think text like that should make it into a standard.
Personally I got used to changing the zoom all the time, with the amount of sites that solve the DPI problem by multiplying the size of everything by 2.
[1] Firefox: https://addons.mozilla.org/en-us/firefox/addon/stylish/
Chrome: https://chrome.google.com/webstore/detail/stylish/fjnbnpbmke...
Find styles: https://userstyles.org/
[1]on iceweasel on debian 8 in virtualbox.
Some websites including Hacker News use very small font size. Reddit's font size is bigger. I created a HN web reader ( http://hackernewsroom.com ) with bigger font size (for myself and friends) to read titles and comments easily. But I can't change the font size of the original article. (Have to zoom in the page by pressing cmd+=)