For HTML export, you can use your own block names which will export as a div
element with the block name as a class:
#+begin_monoblock This is some text. #+end_monoblock
Now you can add your format 2 ways:
1: by adding a CSS style definition for all such blocks at the top of your org document
#+HTML_HEAD: <style>.monoblock </style> ... other things #+begin_monoblock This is some text. #+end_monoblock
2: by adding style or other HTML attributes individually to each block:
#+attr_html: :style font-family:courier; #+begin_monoblock This is some text. #+end_monoblock
Or, a combination of both, with some common style for all blocks, plus extra style individually.