Override crazy browser defaults for h1 fontsize

Fixes #5311

themes/tiddlywiki/vanilla/base.tid

@@ -120,6 +120,15 @@ body.tc-body {
 
 """>>
 
+/**
+ * Correct the font size and margin on `h1` elements within `section` and
+ * `article` contexts in Chrome, Firefox, and Safari.
+ */
+
+h1 {
+	font-size: 2em;
+}
+
 h1, h2, h3, h4, h5, h6 {
 	line-height: 1.2;
 	font-weight: 300;