It gets rather interesting when you see all the ways in which HTML stylesheets can assign text sizes. Here are three:

Pixels:

Probably the most common sizing used among those who design web pages. Each device has a certain number of pixels per inch when it comes to CRT and LCD screens. EInk screens also specify a number of pixels per inch but eInk screens display text differently than a traditional LCD screen. The 350 has appx. 200 pixels per inch of screen both horizontally and vertically. So if you specified a font size that actually stands 20 pixels tall, the text would be 20/200 inches tall or 0.1 inches tall. A PC LCD monitor might have 96 pixels per inch, so the same 20 pixel tall text would stand 20/96 inches or 0.21 inches tall. So pixels can be of different sizes for different screens.

Points:

Probably the most common sizing used in the printing industry where printed books, brochures, etc. are produced. I think points are frequently considered to be 1/72nd of an inch or 72 points per inch. Using points the printing document designers could estimate rather accurately how much text would fit on a printed page. Although I don't recommend using points for eBook stylesheets, a lot of printing based publishers do because that is what they are used to.

EMs:

Instead of using a size per inch, EM is relative. In HTML if you set the standard text size to some amount, then EM represents a size in relation to the standard text size. Setting a size of 1.5em is the same as setting the text size to be 1.5 times (150%) bigger than the standard text size. For example, if you set the standard text size for your document to be 16px, then assigning a value 1.5em to a heading level 1 (H1 tag) will increase the text size for H1s to 24px. In theory, if you do not define the standard text size for a document, the device on which it is displayed will choose one that is optimum for that device. Thus using EMs to define all the headings and non-standard text sizes would make them relative/proportional to the device optimum. In reality that might not actually work out as well as you would like.

Bottomline:

HTML and eBooks are much like the "Life is like a box of chocolates" quote from Forrest Gump, you never know what you will encounter in the stylesheet. I've seen all three used within the same ePub! I personally prefer pixels because I used them so much with HTML in the years gone by. But to be honest, for ebook EMs is probably the more logical form of measurement but perhaps the most difficult to visualize.