Another caveat would be working with responsive design changes. For example on my site, the “Hi” appears on different background colors, depending on the size of the screen. Still, a pretty cool and easy tool to use.
@Jeffrey, that is true! But in this world where more people are moving toward designing/tweaking/iterating in the browser, it can be a really helpful too for those going that route. Plus I like that they provide suggestions to match the criteria, so you can always go back and update your code and QA to see if it meets design qualifications if you missed one or two before the page was built.