> A little googling showed me, that this feature is in Geany wishlist. So, I
> wish to implement it.
> After the brief investigation I've found, that it may be implemented in
> on_update_ui (src/editor.c) by checking active lexer, and if it is HTML,
> then calling editor_highlight_html_tag function (or something like this).

You should do language specific stuff in a plugin, you can connect to
the editor-notify signal. Just ensure you return false.

>
> Is it OK to implement on the editor level, or you think it would be correct
> to implement on the Scintilla lexer level (LexHTML.cxx)? Please, suggest.

I assume you want to make it work like brace matching, when the cursor
is in/next to a tag highlight the opposing tag.

Exactly.

It is probably not possible to implement in the lexer as it won't be
called for changes to the cursor position.

Ok, I will implement it as a plugin. Thank you for the tips!