The self-closing <br/> is always valid. I also use the self-closing version since that will work whereas <br> without the closing tag may or may not work. Many browsers and renderers will accept <br> without the closing tag but others don't. calibre uses <br> but Sigil considers it as an error as does epubcheck. Several publishers will reject an epub with <br> tags that are not terminated when epubcheck barfs it's error messages.
One interesting oddity is that if you place text between the opening <br> tag and closing </br> tag, most renderers will not display it. I got a freebie epub which placed my email address in about 50 different places in the epub using that trick to hide the text from being displayed.
Code:
OEBPS/Text/Section0001.xhtml,10,236,Col: 54: ERROR(RSC-005): Error while parsing file: text not allowed here; expected the element end-tag
OEBPS/Text/Section0001.xhtml,10,238,Col: 56: FATAL(RSC-016): Fatal Error while parsing file: The element type ""br"" must be terminated by the matching end-tag ""</br>"".