I am so glad this is the top comment :)

I checked the site and it's absolutely lovely, I was all ready to jump at the occasion and share this link to all my friends who ask me about "design tips" some times (I got a decade of experience on them so I intuit and spot things that are "off" but often it's first this 4 minute stuff that needs fixed), and when he got to the text #555 bit I was like "NOOOOOOOOOOO now I have to send this to all my friends with the caveat 'but please ignore the text #555 bit'" (which will get lost somewhere and we'll end up with more bad contrast sites).

I can say a bit more about this, but since the site is all about "do this!" (no reasoning given) I'll first just say: "don't do that!" :)

The claim that black text is harsh on the eyes is a similarly unfounded-yet-oft-repeated "Designer Wisdom" like that Golden Ratio shit--it's also similar in the way that it's not per se wrong to use it, but like bbq-sauce on pizza/sex, only if you know wtf you're doing, not something to apply willy-nilly without clue or cleverness.

And it's IMHO slightly worse than the Golden Ratio because 1. the Golden Ratio doesn't hurt and 2. the Golden Ratio thing sounds sciencey but is hard to experimentally confirm or refute while anybody can see that black on any screen was never really black to begin with so what are you on about in the first place "never use pure black" (as a designer, I WISH I could use pure black, but we can't, scientists told me only a black hole is really pure black, and you can't paint with those because they're bad for the environment or something), it's a really arbitrary distinction.

As for "don't use pure black", I do that, but the most important rule of design remains:

You have got to know the rules before you break the rules!

And the rule is black on white. Which you can't achieve. So practice on that first. Then you can break it and ask yourself "why am I breaking it?". Personally what I like to do is use a very dark colour like #321 or I don't know, #114 maybe. This way, on a high quality bright and contrast-rich monitor the people with medium-good eyesight get a slightly coloured tone in the blacks that you can use to match the other colours, warm it up, cool it down etc. And for everybody else, it looks like black! I usually do the same for white. And if you use something like redshift (or Flux) it gets all messed up any way, but if you place your blacks/whites slightly off minimum/maximum and you do it in harmony with the other colours, then a transformation like redshift will also leave a lot of that colour harmony in tact. So that's a good reason not to use pure black or pure white. However, upping your blacks all the way to #555, will get a lot of use out of the non-linearities in the blackbody-radiation that redshift emulates and it'll mess up the colours.

Also #555 is 33%! Post-gamma, even! You just threw away one-third of your dynamic range for what reason? Say somethingcontrastsomethingsomething again, I double-dare you. Again, learn to use contrast first, before you decide to get rid of it because you've heard somethingsomething somewhere.

Otherwise, wow this site is absolutely lovely!! :D Let me not understate that!