support HTML files (see #764)

lib/single-file/single-file-util.js

@@ -94,7 +94,10 @@ function getInstance(utilOptions) {
 			return (new DOMParser()).parseFromString(content, "text/xml");
 		},
 		parseSVGContent(content) {
-			return (new DOMParser()).parseFromString(content, "image/svg+xml");
+			const doc = (new DOMParser()).parseFromString(content, "image/svg+xml");
+			if (doc.querySelector("parsererror")) {
+				return (new DOMParser()).parseFromString(content, "text/html");
+			}
 		},
 		async digest(algo, text) {
 			try {