Treat XHTML without xmlns as HTML

The XHTML 1.1 spec says

    The start tag of the root element of the document MUST explicitly
    contain an xmlns declaration for the XHTML namespace

There doesn't seem to be such a requirement in the WHATWG spec. But at
any rate browsers won't render it properly without the XHTML namespace.

epub.js

@@ -673,8 +673,9 @@ class Loader {
         if ([MIME.XHTML, MIME.HTML, MIME.SVG].includes(mediaType)) {
             let doc = new DOMParser().parseFromString(str, mediaType)
             // change to HTML if it's not valid XHTML
-            if (mediaType === MIME.XHTML && doc.querySelector('parsererror')) {
-                console.warn(doc.querySelector('parsererror').innerText)
+            if (mediaType === MIME.XHTML && (doc.querySelector('parsererror')
+            || !doc.documentElement?.namespaceURI)) {
+                console.warn(doc.querySelector('parsererror')?.innerText ?? 'Invalid XHTML')
                 item.mediaType = MIME.HTML
                 doc = new DOMParser().parseFromString(str, item.mediaType)
             }

mobi.js

@@ -1154,7 +1154,7 @@ class KF8 {
 
         // by default, type is XHTML; change to HTML if it's not valid XHTML
         let doc = this.parser.parseFromString(replaced, this.#type)
-        if (doc.querySelector('parsererror')) {
+        if (doc.querySelector('parsererror') || !doc.documentElement?.namespaceURI) {
             this.#type = MIME.HTML
             doc = this.parser.parseFromString(replaced, this.#type)
         }