Writing well-formed HTML is really quite simple. It has the benefits of markup being unambiguous as well as a way to increase the robustness of the Web, while simplifying and accelerating the processing of well-formed documents and data.

It is good to choose a consistent case for open and close tags, though this is not enforced as in case of XML where tags are case sensitive. There is no recommendation to use either upper or lower case, as long as it is consistent between start and end tags. Generally use upper case for HTML elements for consistency:

Aviod mixed tags
Preferred usage of tags