This isn’t supported officially. Going back to CSS1, images can be set as list markers, and all kinds of numbering styles can be chosen, but picking a character as marker of an unordered list isn’t part of the specs. Pity, there are nice characters in UTF for this purpose, like this one:
☞
The workaround that I found is to set the list-style-type to “none”, and then prefix the character to the list items with the content property, like this:
ul {list-style-type: none;}
ul li:before {content: "☞";}
Unfortunately, there is no way to directly make it display like a list marker either. This can be done in a not very elegant way by setting a negative margin:
ul {list-style-type: none;}
ul li:before {content: "☞"; margin-left: -1.2em;}
I found that 1.2 em works near perfectly for this specific character. For other characters, you’ll have to experiment.