_isBasicCustomElement = function _isBasicCustomElement(tagName) {
      return tagName !== 'annotation-xml' && stringMatch(tagName, CUSTOM_ELEMENT);
    }
_isBasicCustomElement
checks if at least one dash is included in tagName, and it's not the first char
for more sophisticated checking see https://github.com/sindresorhus/validate-element-name

@param tagName name of the tag of the node to sanitize
@returns Returns true if the tag name meets the basic criteria for a custom element, otherwise false.