Support variation of HTML5 signature:

<!DOCTYPE html SYSTEM "about:legacy-compat">

This is an alternative for HTML generators that don't support DOCTYPE html.  XSLT is an example of such a generator.
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5
Merge branch 'HtmlViewer-11.4-to-11.5' of https://github.com/BerndGabriel/HtmlViewer into HtmlViewer-11.4-to-11.5