Your website probably has a color-scheme. Green on white, purple on yellow, whatever. When you give users the ability to style their content, you may want them to keep in line with your styling. If you're website is all about light colors, you don't want a user to come in and vandalize your page with a deep maroon.
This is an extremely silly feature proposal, but I'm writing it down anyway.
What if the user could constrain the colors specified in inline styles? You are only allowed to use these shades of dark green for text and these shades of light yellow for the background. At the very least, you could ensure that we did not have pale yellow on white text.