ignore HTML served as CSS (fix #665, fix #663, fix #661)

lib/single-file/single-file-core.js

@@ -1590,6 +1590,9 @@ class ProcessorHelper {
 						const content = await getStylesheetContent(resourceURL);
 						resourceURL = content.resourceURL;
 						content.data = getUpdatedResourceContent(resourceURL, content, options);
+						if (content.data && content.data.match(/^<!doctype /i)) {
+							content.data = "";
+						}
 						let importedStylesheetContent = removeCssComments(content.data);
 						if (options.compressCSS) {
 							importedStylesheetContent = util.compressCSS(importedStylesheetContent);
@@ -1694,6 +1697,9 @@ class ProcessorHelper {
 			}
 			resourceURL = content.resourceURL;
 			content.data = getUpdatedResourceContent(content.resourceURL, content, options);
+			if (content.data && content.data.match(/^<!doctype /i)) {
+				content.data = "";
+			}
 			let stylesheetContent = removeCssComments(content.data);
 			if (options.compressCSS) {
 				stylesheetContent = util.compressCSS(stylesheetContent);