I really didn't code in HTML for a while, so I'll give u a general idea.

-First, use an online or any tool to reformat/beautify your chaotic html
file.
-After the file is readable, try to find which fonts the site uses (You can
look for 'font-family' in the style block, or use a web extension like
WhatFont, in the worst cases, just assume it's using a standard font like
'sans-serif' or any one you prefer)

-Now, download the fonts that the site uses (there are many sources for
that).
-Now, either locate the main CSS file, and add this to it:

@font-face {
  font-family: 'MyLocalFont';
  src: url('path/to/your/font.ttf') format('truetype');
}
body {
  font-family: 'MyLocalFont' !important;
}


-Or locate your HTML file, create a style block in the head section and add it
the same way.

-If you have many HTML/CSS file, u might need to create a script (like a
python script) to automate this.

This is all I know for now...