diff --git a/src/document/html/internal.h b/src/document/html/internal.h
index a28aeb93..89a56df1 100644
--- a/src/document/html/internal.h
+++ b/src/document/html/internal.h
@@ -66,7 +66,8 @@ struct html_context {
 					   * state-machine. */
 	int was_li;
 
-	int quote_level; /* Nesting level of <q> tags. */
+	unsigned int quote_level; /* Nesting level of <q> tags. See @html_quote
+				   * for why this is unsigned. */
 
 	unsigned int was_br:1;
 	unsigned int was_xmp:1;
diff --git a/src/document/html/parser/general.c b/src/document/html/parser/general.c
index 1184bf2b..33fe08c0 100644
--- a/src/document/html/parser/general.c
+++ b/src/document/html/parser/general.c
@@ -104,6 +104,14 @@ void
 html_quote(struct html_context *html_context, unsigned char *a,
 	   unsigned char *xxx3, unsigned char *xxx4, unsigned char **xxx5)
 {
+	/* An HTML document containing extremely many repetitions of
+	 * "<q>" could cause @html_context->quote_level to overflow.
+	 * Because it is unsigned, it then wraps around to zero, and
+	 * we don't get a negative array index here.  If the document
+	 * then tries to close the quotes with "</q>", @html_quote_close
+	 * won't let the quote level wrap back, so it will render the
+	 * quotes incorrectly, but such a document probably doesn't
+	 * make sense anyway.  */
 	unsigned char *q = quote_char[html_context->quote_level++ % 2];
 
 	put_chrs(html_context, q, 1);