I'm using the following line
const doc = new DOMParser().parseFromString(html, 'text/html');
and realize
if html = '1<br/>2', the result doc only contains '<br/>2'.
if html = '1<br/>2<br/>3', the result doc only contains '<br/>2<br/>3'.

Seems like if <br/> is the first tag, then any text content before the first <br/> is ignored.

Notice if you have some tag wrap that text content it can maintain (In other words, <br/> is not the first tag any more)
if html = '<b>1</b><br/>2', the result doc is '<b>1</b><br/>2'.