Especially the ones that use a very light grey font? Here's something that may help you. And this is specifically for folks on Windows computers. I'm not familiar with the Apple system.
First, go to a website that you have trouble reading. Minimize your browser so your desktop is showing and right click a blank area. Click "graphics properties". Click "display". Click "color settings" (on the left). At the bottom of the frame look at your gamma settings. The default will probably be set at "1". Reduce the gamma setting lower. Small increments can make big changes. Go down 1 # at a time and check the web page you were on. Keep going until you find a setting that makes things easier for you to read. Then save your settings. Don't go too far or you'll darken things so much it will negatively affect graphics and photos. I found going to 1.0 to 0.8 did the trick for me.
Maybe one day web site developers will think about those of us with vision problems.