The demo page is a representative case of the "designer font-size" setting. I get exactly 4 lines of text on the first pageful. Not everyone surfs on Retina displays or humongous iMac panels at 1080 vertical resolution. As much as I appreciate being able to admire individual glyph curves up close and personal, it comes at the expense of being actually read the page without constant scrolling.
For whatever reason, most sites seem to err on having fonts that are far too small. I would much rather they err on having fonts too large, and I do not think this particular page erred at all.
Perhaps it doesn't work well on a cell phone or tablet, but it works perfectly fine on any even moderately modern laptop or desktop.
As an aside, I really love the GitHub and Twitter icons. Most sites have icons that don't quite match each other or the overall design; these icons match both all the while being very simple and elegant.
That said, I agree that a base font size of 18px is too large.
Now personally I believe you could probably shrink the font size a little and still look good, but you still wouldn't get much content on the first page without fixing everything else I mentioned.
I've always found the opposite; because headings are typically larger, they benefit from a lighter colour than the body copy.
The W3C states a minimum of 4.5:1 contrast ratio for body copy, or 7:1 for users with poor eyesight. If Snook's calculator can be relied upon, this makes #444/#fff a contrast of 9.74:1. Even a darker bg of #eee yields 8.39:1.
This also depends on the font weight, stroke, and size.
Better be safe and go with a darker text.
But darker text isn't necessarily safer, because having too much contrast is also a bad idea. That is why these guidelines about not using pure black or pure white get passed around so much in the first place.
Only dumb-ass designers have asserted that grey-text-on-grey-backgrounds is in any way a good thing, and they are wrong. (At least as far as conveying information goes.)
As someone who does read a lot of research about design and usability issues, I am always sceptical of people who write things like "There are hundreds of studies to that effect" yet don't cite a single one. If it were really as simple as you claim, we would all be reading white or yellow text on black screens and no-one would get headaches because their monitor was set too bright.
If you really do believe in looking at empirical data, you might like to read some of the research into how excessive contrast can exacerbate negative conditions, such as the "shaking" effect seen particularly by dyslexics or the "halo" effect seen particularly by older readers.
A few places I inspected the elements to see how it was working in the real world - i.e. this site - and it's a bit of a pain to work out which bit of line 29 a particular rule is on.
Such things are not required in digital media where each reader might have completely different spaces for text to render within. And, ideally, the content and its formatting are structured in such a way as to allow the user to apply their own formatting if desired.
warning & disclaimer: post may contain factually incorrect guesses and suppositions of the author; no information herein should be considered accurate or factual; use at your own risk
Then along came Microsoft Word, which inserted vertical space between paragraphs instead. Now most people think it's supposed to be that way.
I have no citation for this, but have noticed that novels in print use indentation rather than spacing.
the earlier tradition was that you'd add a pilcrow or some other distinctive mark at the start of a logical paragraph mid-line so that as little paper (which was either baby cow parchment or made of precious linen) was wasted on whitespace. For an example, look at a 42 line bible  or eric gill's book on typography .
Other great books: Ellen Lupton's Thinking With Type and James Felici's The Complete Manual of Typography. Both are really solid.
Books that would be worth checking out at a library: Richard Hendel's On Book Design and Ruari McLean's Typographers on Type. More philosophical in nature, less practical.
It's a classic typography book translated to a web context with the relevant CSS styles. Pretty cool.
For a more easily digestible (and interactive!) overview, the one that comes to mind is the Interactive Guide to Blog Typography: http://www.kaikkonendesign.fi/typography/
If you'd like to check out more resources, hackdesign, a newsletter of curated design-related links, is focusing on typography this month: http://hackdesign.org/courses/
Can somebody shed light on the recent trend for huge font sizes? Not guesses and anecdotes but some research?
The margins up and down the headings don't seem to be consistent though. For the big ones they are much too small, for the small ones way too big (when combined with paragraphs). Also, if they are below
's, margins above the heading are larger than below (big sin in typography). I don't know if margins are defined somewhere in Typeplate, maybe that'd be an issue to solve.
I also don't quite understand the way, font-sizes and scales are measured/calculated. It would be nice if you could document that so designers without extensive coding skills are able to understand and change that system to their needs. Are all font-sizes based on em? Is there any simple way to change the default font-size? Changes on the variables don't seem to have an effect for me..
Besides those (for me) issues this is really great stuff, thanks for sharing!