HTML must have an end of /html to be a complete element, unless it is a self-closed tag. Currently, most websites use the xhtml standard writing method for SEO and multi-browser support. According to the principle of w3c, each start tag of xml must have an end tag corresponding to it, that is, <html> must have a end of </html> to be a complete element, unless it is a self-closed tag, self-closed is a label such as <img src=... />.
HTML is a child of xml, so you must also follow the principle of xml, which is the standard.
Therefore, <meta charset=UTF-8> is not added to the standard, and it does not meet the standards. It is only suitable for versions before html 4.0. Currently, most websites use the xhtml standard writing method for SEO and multi-browser support.