This question has been asked on SuperUser and StackOverflow, but I think having it here too is okay. Its something I never even considered.
The answer appears to be to use Phantom.js or one of the websites people have already built with it:
I haven't used any Phantom methods before, but might have to remember this.
Another option, which I have personally used, is magnifying the page (Ctrl / Cmd + + and taking screenshot(s) that way, then combining them into one large image as necessary.
In the magnifying of page you'll have to be weary of raster images, they will get pixelated which might defeat the purpose. I'm not sure how it works with the services, I imagine the same since I don't know how it would convert raster into a vector format. I'll test this when I get to my office and edit to confirm.