When computing font size, I know that 16px equals 1em. What I need to know is: How many pt equals 1px?
The relationship between pt and px varies according to the user's operating system settings. For example, setting "large fonts" in the Windows display settings increases the pixels per pt.
Technically, pt is 1/72 of an inch, but the number of pixels per inch on computer screens vary hugely. An old 15" CRT usually displayed 800 x 600, but a modern laptop display of the same physical size might be 1440 x 1200 px. In order to stop text being unreadably small, OS defaults have changed over the years. Windows 95 and 98 assumed 72dpi. Now it is 96 and even 120 dpi.
The bottom line is that pt, like mm and inches, should only be used for print CSS.
ems are scalable in that as the user's font size gets larger, so do the containers. If you are trying to create a fixed width layout (either in px or %) to avoid horizontal scrolling, that puts pressure on the layout, especially if only one column is left "fluid". Which is worse, having to scroll horizontally or having one-word-wide columns?
Sizing in px retains more designer control but is less user-friendly, especially at 144dpi. And if the user has to resort to zoom functions to view the site, they will once again end up having to scroll horizontally.
pt sizes differently. 20pt in one browser could be another size in a different browser. That is why most people size fonts in px or em.
pt is generally used for printed media, while px (or em, or %) are used for screen text
As others have said pt is for print stylesheets, but an approximate guide found here might help.
12pt is 16px is 1em is 100% (roughly and in some browsers)
just for reference. 72pt is the same as 1 inch