You are building an HTML5 site, not a XHTML site with the app.
HTML5 DocType - <!DOCTYPE html>, closing tags not required.
XHTML DocType - <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">, closing tags ARE required.
XHTML is stricter than HTML(5) - https://www.w3schools.com/Html/html_xhtml.asp
However, I do understand your thought that if it's in the app like that you'd expect the export to also to be like that.