When I edit an HTML document, Geany's syntax highlighter allows
``` <!DOCTYPE html> ```
But if I write 'doctype' in lowercase, the syntax highlighter marks 'doctype' in red. According to the HTML specification, 'doctype' is not case-sensitive:
https://www.w3.org/TR/html51/syntax.html#the-doctype
So the syntax highlighter should allow this.
You need to change the `sgml` keywords in `filetypes.html` to add a lower-case `doctype` to the list (tested, it works). Probably none of those tags should be required to be only upper-case, but it would require fixing the HTML lexer from Scintilla to change that.
Refer to the user manual for information about customizing filetypes if you haven't done it before.
In all HTMLs and XHTMLs before HTML5 and in XHTML5 it is required to be upper case. As the lexer that highlights html is not sensitive to the version it can't change its habits based on the version.
The lexer itself comes from the [Scintilla](http://www.scintilla.org/) project so you would need to ask for it there to make it vary by version.
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