Open a empty writer page.
Save it as "HTML".
Have a look at the HTML-code.
It will look like this:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<meta http-equiv="content-type" content="text/html; charset=utf-8"/>
<meta name="generator" content="LibreOffice 18.104.22.168 (Linux)"/>
There is a DOCTYPE HTML 4.0 (which is obsolete since 1999!) and every standalone tag ends with "/>", which is only allowed with HTML5 or XHTML.
Two possible solutions:
1) Set <!DOCTYPE html> instead of the very old DOCTYPE. It will set the document to HTML5.
2) Remove all frontslashes followed by ">".
I would prefer to go forward, not back to 1999!
All tested with LO 22.214.171.124 (see above), OpenSUSE 15 64bit rpm Linux.
confirmed with LO 126.96.36.199 (Ubuntu Mate 18.04)
Could also reproduce this in OpenSUSE 15 with LO 188.8.131.52, the oldest Version, I have installed here.