Preexisting htmls are not embedding correctly (raw:: html)

Describe the bug

My documentation largely depends on the raw:: html method of embedding preexisting html files. For example:

.. raw:: html
   :file: ../another_directory/to_embed.html

However, the rendering of the embedded file looks VERY different from the original html file. Here is a snapshot of one of the sphinx rendered embedded htmls:

How to Reproduce

$ git clone https://github.com/iqcToolbox/iqcToolbox.git
$ python3 -m pip install sphinx_rtd_theme
$ cd iqcToolbox/docs
$ git checkout docs-003-delta_user_documentation
$ make html 
$ firefox build/html/user.html 
$ firefox mat_html/userUlft.html

The last firefox command shows you what you should be seeing, whereas the 2nd to last shows you what sphinx is producing.

Expected behavior

To compare against the previously posted image, this is what the original html produces:

Your project

https://github.com/iqcToolbox/iqcToolbox/tree/docs-003-delta_user_documentation/docs

Screenshots

No response

OS

Ubuntu 20.04

Python version

3.8.10

Sphinx version

1.8.5

Sphinx extensions

No response

Extra tools

No response

Additional context

No response

Reproduce the issue from the iqcToolbox docs checkout using the raw:: html directive, make html, and the generated build/html/user.html, then compare it with mat_html/userUlft.html. Trace how the preexisting HTML file is embedded and determine what would make the rendered result match the original HTML; done means the two renderings agree.