VIM syntax highlighting of html nested in yaml

Question

Given a yaml file that contains html, like this:

template        : |+ 
    
Hello, world

Is

Answer 1

I used Maxy-B's solution. My code, in particular, is a bit different so I thought to post it for posterity:

~/.vim/after/syntax/yaml.vim

let b:current_syntax = ''
unlet b:current_syntax
syntax include @HTML syntax/html.vim

syntax region htmlCode start=#^html:#hs=e+1 end=+^\w+he=s-1,me=s-1
    \ contains=@HTML

let b:current_syntax = ''
unlet b:current_syntax
syntax include @TEX syntax/tex.vim

syntax region texCode start=#^tex:#hs=e+1 end=+^\w+he=s-1,me=s-1
    \ contains=@TEX

This highlights the top-level YAML nodes html and tex with those respective types of code. It's not very dynamic, but it suits my purpose and this may serve as helpful guideline for someone doing something similar. It'll highlight the following as expected (or at least, as I expect it), for example:

regular:  # yaml
   - yaml # yaml
html: 
    hello # html
tex:
    \begin{document} # tex
    \end{document} # tex
the-end: may be necessary # yaml