HTML5 specifies two variants of the HTML5 doctype:
Common: <!DOCTYPE html>
Uncommmon: <!DOCTYPE html SYSTEM "about:legacy-compat">
The purpose of the uncommon variant is to silence (XML) tools that refues to produce a DOCTYPE without a URI. It is not supposed to have any effect that is different from the common variant. (I use the uncommon variant now and then because I have an - otherwise exellent - XML editor that otherwise makes a lot of noise. I have also found the uncommon doctype in online documents.)
However, in BlueGriffon, the uncommon variant triggers HTML syntax generation - as opposed to XHTML syntax generation. And the trigger effect overrides other signals (such as the XML prologue (<?xml version="1.0"?>) or the XHTML namespace declaration) that for documents with the common DOCTYPE, triggers XHTML generation.
Exampe - do this:
1. In some editor, create a valid XHTML5 file:
<!DOCTYPE html SYSTEM "about:legacy-compat">
<html xml:lang="en" lang="en" xmlns="http://www.w3.org/1999/xhtml">
2. Save the file with the .xhtml file suffix
3. Open the file in BlueGriffon.
4. Check the source code
Same behavior as when using <!DOCTYPE html>:
That is: BlueGriffon produces XHTML syntax.
BG produces HTML5 syntax thatis both non-XML and non-polyglot.
Please make it irrelevant whether the common or the uncommon doctype is used.
Fixed and checked in.