Make HTML behave exactly like other content formats (note)

Fixes #11999

parser/pageparser/pagelexer_intro.go

@@ -13,10 +13,6 @@
 
 package pageparser
 
-import "errors"
-
-var ErrPlainHTMLDocumentsNotSupported = errors.New("plain HTML documents not supported")
-
 func lexIntroSection(l *pageLexer) stateFunc {
 	l.summaryDivider = summaryDivider
 
@@ -39,19 +35,6 @@ LOOP:
 		case r == byteOrderMark:
 			l.emit(TypeIgnore)
 		case !isSpace(r) && !isEndOfLine(r):
-			if r == '<' {
-				l.backup()
-				if l.hasPrefix(htmlCommentStart) {
-					// This may be commented out front matter, which should
-					// still be read.
-					l.consumeToNextLine()
-					l.isInHTMLComment = true
-					l.emit(TypeIgnore)
-					continue LOOP
-				} else {
-					return l.documentError(ErrPlainHTMLDocumentsNotSupported)
-				}
-			}
 			break LOOP
 		}
 	}
@@ -60,19 +43,6 @@ LOOP:
 	return lexMainSection
 }
 
-func lexEndFrontMatterHTMLComment(l *pageLexer) stateFunc {
-	l.isInHTMLComment = false
-	right := l.index(htmlCommentEnd)
-	if right == -1 {
-		return l.errorf("starting HTML comment with no end")
-	}
-	l.pos += right + len(htmlCommentEnd)
-	l.emit(TypeIgnore)
-
-	// Now move on to the shortcodes.
-	return lexMainSection
-}
-
 func lexFrontMatterJSON(l *pageLexer) stateFunc {
 	// Include the left delimiter
 	l.backup()