Honestly, this is why over time I've been longing to return to print design. Or try my hand at iOS app design. We can talk about graceful degradation all we like, but if somebody is viewing a site I've designed on shitty hardware, they're going to have a shitty experience.

The only solution I can think of to this issue is to bump up the contrast of your colour scheme, but then you've sacrificed subtlety in order to please a lowest common denominator. Accessibility-wise, that makes total sense, but it's frustrating!

This very page you're looking at is a pretty good example. On my Mac screen, the blue-on-blue header colour scheme looks beautiful. On a crappy PC screen, though, I imagine it would be virtually impossible to differentiate the text from background.

This is definitely something to keep in mind. Just the same that pro audio guys test big releases on terrible speakers and sound systems as most people won't be hearing it on $100k audio setups.

One note though regarding your flat design comment. The issue isn't flat design with its lack of gradients, but rather the issue here is contrast. A subtle gradient won't ready any better than flat design.

With design for screen, you have to give up some control and be comfortable with that. You should always design for as much of your audience as possible, but its also worth keeping in mind that you can't optimally design for everyone.

This is why I still design on a Windows machine and check mock-ups on multiple other devices including Retina Mac.

I highly suggest signing up for a free account on http://www.gosquared.com. It will give you a full analysis/breakdown of all of the data with your site and will also tell you what resolution size your users have.

Crappy screens aren't the only problem we should be checking. On a low contrast design like this there will be issues in even different lighting. With the prevalence of tablets and smartphones it is time to start taking our designs for a walk outside and seeing how they look in the sunlight instead of the dark studios lit by the glow of our 27" Cinema Displays.

I've been doing some responsive designs for a news site and have had to consider the contrast of text & content especially on mobile as our users are going to be reading on the go with various screens and a multitude of lighting conditions.