Directory separator is os-dependent ('/' vs. '\', css_file)

[Sogolumbo]

The path to the css file may have to be interpreted by a browser on a windows machine. The hardcoded / characters here cause an error:
The css file can not be found.

Replacing the line with the following code works on Windows:

css_file = "file:///" + os.path.join(plugin_path, "static", "seo_report.css")

[MaevaBrunelles]

Indeed. I suppose that the fix you propose couldn't work on Unix with "file:///". I need some time to do some tests.