Does not recognize HTML files
GNOME builder does not seem to recognize any HTML files. The files are perfectly valid (syntax) and do not have any issues. It is also correctly recognized as HTML by GNOME Builder and syntax-highlighting works.
You can find the file I used here: https://github.com/rugk/offline-qr-code/blob/master/src/options.html
It shows errors like "No declaration for element HTML", which looks like some XML parsing issues.
GNOME Builder v3.26.4 Fedora 27