As I mentioned in a comment, section Exporting code blocks describes permissible values of the :exports option: code (contents of the block), results (results of its evaluation), both (code and results) and none (nothing is exported).
By default Org tries to interpret results of evaluating a source code block (and turn it into a table, for example). If such an interpretation fails, evaluation results are formatted as monospace text and look as if wrapped in <pre> ... </pre> when exported to HTML.
Luckily we can tell Org to put evaluation results in a special kind of a block using :results (see Results of evaluation and :results). If you want to be able to use a source code block to generate a part of an HTML page, use :results value html or :results output html and Org will wrap results in a #+BEGIN_HTML ... #+END_HTML block.
Here's a minimal example (#+OPTIONS are irrelevant, I added them to make it easier to inspect exported HTML code using C-c C-e h H):
#+OPTIONS: html-postamble:nil
#+OPTIONS: html-preamble:nil html-scripts:nil html-style:nil
#+OPTIONS: html5-fancy:nil tex:t
Some text.
#+BEGIN_SRC python :exports results :results value html
return "<hr> Generated HTML code. <hr>"
#+END_SRC
More text.