The question is basically - what do they actually mean? I have just been playing with setting some font sizes to various 'relative' sizes and the results don't seem to make sense to me.
OK, I *think* I know what the 'absolute' sizes mean. If I set a size to "1cm" (yes, I know the quotes aren't necessary in the CSS) I get something that is 1cm high and if I set it to "1in" I get text one inch high. However I'm not at all clear how the system knows how big my display is, if I connect a smaller display to the same output then the text will be smaller even though I have changed nothing. Is the correct absolute size dependent on me having set up my display correctly at some point?
Anyway, on to the bit I understand less well, what do relative sizes mean? For font-size we have:-
* em (ems, the height of the element's font) * ex (x-height, the height of the letter "x") * px (pixels, relative to the canvas resolution)
So what is meant by "the height of the element's font"? It can't be an absolute value (in cm or in) because that would mean the result would be absolute. I can see that there could be a relation between everything and, say, the default BODY text size so that if you change the BODY text size everything else changes in proportion but where does that BODY text size come from? Is it what I set in my browser as the font size?
If that is right I sort of follow, everything is relative to the default font size set in the browser. Sizes set in em or ex are then relative to that size. However that leaves pixels, what on earth does "pixels, relative to the canvas resolution" mean? Does it mean a size related to the screen size, but how does one interpret 'pixels' in this context? If I say the font size is to be 16 pixels what does that really mean? If it really means 16 pixels of my screen then surely it's an absolute size.
Certainly none of the relative sizes are relative to the browser window size which in many ways would make most sense. If they *were* relative to the window size a web page would (if it used relative sizes) just scale as you grow and shrink the window. As it is, even with relative sizes for things, when you grow and shrink the window the text stays the same size and is re-laid out to fit.
Whatever, there's surely a web site somewhere that explains all this, can someone point me at it please. http://www.htmlhelp.com/ is an excellent site for the details of writing HTML and CSS but doesn't really explain the sizing (not for me anyway).