One of Vim's great strengths is object-select, offering quick manipulation of content inside words, paragraphs and assorted delimiters.
For example,
vi{
will select everything inside a pair of {} braces.
Is there any equivalent functionality for selecting a here document or heredoc:
<<<HTML
....
....
HTML;
For selecting heredoc's I usually place the cursor at the first line, over the heredoc identifier and press V*
V will start a line selection, and * will start a search, going to the next match of the identifier, the end of the heredoc...
A <<'' heredoc terminated by an empty line is easy, if you're already at the start (?<<^M^M): v} selects from here until the empty line.
Otherwise, in your example the best I can think of is v/^HTML.
There's this plugin that let's you define your own text objects.
http://www.vim.org/scripts/script.php?script_id=2100
I imagine it would be nice to define one so you could say 'yih' (yank in heredoc) so you don't have to explicitly go to the start.
Note I haven't fooled around with this myself.
If you just want a visual select you could.
nnoremap <leader>ih ?HTML<cr>V/HTML<cr>