gh-134580: Modernizing difflib.HtmlDiff for HTML Output

[Wulian233]

Improved the styling of HTML diff pages generated by the difflib.HtmlDiff` class, and migrated the output to the HTML5 standard.
And all browser warnings have been eliminated.

details see: #134580

Request review @hugovk thanks!

Screenshot:

Before:

After:

Before:

After:

• Issue: Modernizing difflib.HtmlDiff for HTML Output #134580

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📚 Documentation preview 📚: https://cpython-previews--134581.org.readthedocs.build/

[hugovk]

Thanks, I think this is an improvement.

Before

After

And

Here's a GitHub diff:

Similarly, I think we should keep the lines a bit tighter together.

Here's the PR with the vertical padding removed:

Shall we remove that padding?

And shall we centre the table on the page?

[Wulian233]

Similarly, I think we should keep the lines a bit tighter together.
Here's the PR with the vertical padding removed:

It's a good improvement, and this is what it looks like now

And shall we centre the table on the page?

I before thought about this, but the table width is determined based on the length of the content and the code specified, like

cpython/Lib/test/test_difflib.py

Line 227 in 1288aa1

'<h2>tabsize=2</h2>',

             '<h2>tabsize=2</h2>',
             j.make_table(f2,t2),
             '<h2>tabsize=default</h2>',

If we center the table, there will be a lot of space on both sides of the screen, and it may not be as easy to distinguish as if the table is narrow. So I didn't do that, and my opinion is also to keep the left alignment the way :)