HTML is rather complex and I'm not sure even a markdown renderer is in the scope of a gui library.
This request consists of 2 phases to me:
1) Create some kind of RichTextLabel that allows rendering text with different colors and text styles.
2) Implement an HTML or markdown widget that uses the RichTextLabel internally.
Although I would love to have both of these things in TGUI, it is very unlikely that the second phase will be implemented unless someone contributes it.
The RichTextLabel is something that is already on the TODO list, but it also shouldn't be expected anywhere soon.