Make it easier for users to see and hear content including separating foreground from background.
Source: WorldWideWeb Consortium Web Accessibility Initiative
Disclaimer: In order to illustrate the point, this page is in complete violation of this guideline. It is my hope that those with visual impairments are using appropriate assistive technology.
Here is a bit more information about accessibility that might interest you.
While the previous cache illustrated how using an image to convey important information is biased against people who are blind, visual impairment does not necessarily mean blind; there are a number of different types of visual impairment. For instance, it can mean the inability to distinguish between colors that don't have enough contrast. I, for one, was never able to see the number in the color plate test when I went to the eye doctor. I am, colloquially, color-blind, though that is not technically true. I would more appropriately be classified as color deficient. And, believe it or not, there are still instances of poor design that cause me to not be able to see what I'm supposed to be able to see. Recently when I was using one of those card reader devices to check out at a store, I was presented the option to receive a printed receipt, an email receipt or both. I opted to receive an email receipt. The confirmation screen for my email address wanted me to choose one or another button to indicate whether it was correct. I was utterly unable to detect which button was "Yes" and which one was "No" because of the poor color choices and combinations. I needed to ask the cashier to make the correct choice for me.
If you look below this paragraph it appears that there is a blank screen. But, the answer is literally right in front of your face right now; you just can't see it. I have intentionally made the text indistinguishable.