One of the most irritating “web design” trends of late is using low-contrast gray fonts for body text. That might be okay for headings or short blocks of advertising copy, but you really shouldn’t do this for large blocks of text. Why do you want to make your text more difficult to read? What, exactly, could you possibly gain by that?
There’s a reason why most books use solid black text. It’s not because of cost. Color ink costs more, that’s true, but gray text could be produced by by printing black ink more thinly, so it should actually be cheaper. The fact that no book publisher does this (again with exceptions for headings, sidebars, or the like) should give you pause. Just stop. My eyes thank you.
Update: it just occurred to me that some of this might be an ill-advised attempt to reproduce the look of Apple material. Now, Apple does use gray font colors in short advertising paragraphs, headings, and so on (just as I noted above), but go look at some long-form material from Apple (developer documentation, e.g.). What color is the text? Hint: not gray.
Update #2: I got a few nitpicky comments in other venues about the fact that this site itself does not use stark black and white. I would have thought that it was clear from context that what I was complaining about was low-contrast gray text of the type shown in the example, but apparently some people need to have that made explicit.
If you really want to get picky, no monitor can display pure black or pure white anyway. This text is legible. This text is not.