syntax: html: colorize specially the other two emphasizing tags too

In most cases, <em> behaves like <i> and <strong> behaves like <b>,
so it makes sense to treat all of them in the same manner.

syntax/html.nanorc

@@ -8,8 +8,8 @@ formatter tidy -m -q
 
 # Tags:
 color cyan "<[[:alpha:]/!?][^>]*>"
-# Bold, italic, and underlined:
-color brightmagenta "</?[biu]>"
+# Bold, italic, underlined, emphasis, and importance:
+color brightmagenta "</?(b|i|u|em|strong)>"
 
 # Named character references:
 color red "&[^;[:space:]]*;"