Personally, I like to use named entities when they are available, because they make my HTML more readable. Because of that, I like to use ✓
for ✓ and ✗
for ✗. If you're not sure whether a named entity exists for the character you want, try the &what search site. It includes the name for each entity, if there is one.
As mentioned in the comments, ✓
and ✗
are not supported in HTML4, so you may be better off using the more cryptic ✓
and ✗
if you want to target the most browsers. The most definitive references I could find were on the W3C site: HTML4 and HTML5.
✗
✗
✘
✘
✕
✕
✖
✖
It's between the Z and the C on your keyboard.
Source: Stackoverflow.com