What causes this graphical glitch in my web browser?

Your screenshot is displaying a repeating pattern of video artifacts which is characteristic of video RAM artifacts.

If your computer screen is displaying video RAM artifacts, then it would show up first in web browsers like Internet Explorer, Firefox and Google Chrome because these web browsers use a lot of RAM.

If the RAM in your graphics card is failing, sometimes just a small fraction of the RAM goes bad while the rest of the RAM continues to work properly. This can result in a screen that looks like a fraction of the screen has glitchy repeating patterns of video artifacts, while the rest of the screen is rendered properly. As a workaround in Chrome, browse to Settings -> Show advanced settings... -> scroll down to the System heading and uncheck the checkbox to the left of Use hardware acceleration when available.

Hardware acceleration in Chrome can also be temporarily disabled from the command line in Windows by starting Chrome using the --disable-gpu Google Chrome command line switch as follows:

1. Open a cmd window.

2. Use cd to change the directory to your Chrome profile directory. In Vista and Windows 7 or newer it is %LOCALAPPDATA%\Google\Chrome\Application

   cd %LOCALAPPDATA%\Google\Chrome\Application
   

3. Type the following command and press Enter

   chrome.exe --disable-gpu