H1, H2, H3, H4, H5, H6 inside table TH tags? - HTML

The HTML living standard explicity forbids this. The th element content model is as follows (emphasis added):

Flow content, but with no header, footer, sectioning content, or heading content descendants.

"Heading content" refers to the h1, h2 etc. elements, as well as the hgroup element:

Heading content defines the header of a section (whether explicitly marked up using sectioning content elements, or implied by the heading content itself).

h1 h2 h3 h4 h5 h6 hgroup