HTML end tag issues and w3c standards_HTML/Xhtml_Web page production

According to the principles of w3c, each start tag of xml must have a corresponding end tag, that is, must end with to be a complete element, unless it is a self-closing tag, self-closing It’s a tag like .
html is a child of xml, so it must also follow the principles of xml. This is the standard.
So, does not add/> to end, it does not meet the standards and is only suitable for versions before HTML 4.0. Currently, most websites use the xhtml standard writing method for SEO and multi-browser support.