diff --git a/publish b/publish
index 6fc7cbd..603c870 100755
--- a/publish
+++ b/publish
@@ -25,6 +25,11 @@ rm -f examples/*.pyc
 cp -R examples build/
 cp .htaccess build/
 
+echo "validating HTML"
+for f in *.html; do
+  python3 util/validate.py "$f" > /dev/null || die "Failed to validate $f"
+done
+
 echo "building HTML distribution"
 htmlbasedir=diveintopython3-r"$revision"-"$today"
 htmldir=build/"$htmlbasedir"
diff --git a/regular-expressions.html b/regular-expressions.html
index f384049..5be0d59 100755
--- a/regular-expressions.html
+++ b/regular-expressions.html
@@ -84,7 +84,6 @@ body{counter-reset:h1 5}
 </ul>
 <p>The following are some general rules for constructing Roman numerals:
 <ul>
-<ol>
 <li>Sometimes characters are additive. <code>I</code> is <code>1</code>, <code>II</code> is <code>2</code>, and <code>III</code> is <code>3</code>. <code>VI</code> is <code>6</code> (literally, &#8220;<code>5</code> and <code>1</code>&#8221;), <code>VII</code> is <code>7</code>, and <code>VIII</code> is <code>8</code>.
 <li>The tens characters (<code>I</code>, <code>X</code>, <code>C</code>, and <code>M</code>) can be repeated up to three times. At <code>4</code>, you need to subtract from the next highest fives character. You can't represent <code>4</code> as <code>IIII</code>; instead, it is represented as <code>IV</code> (&#8220;<code>1</code> less than <code>5</code>&#8221;). <code>40</code> is written as <code>XL</code> (&#8220;<code>10</code> less than <code>50</code>&#8221;), <code>41</code> as <code>XLI</code>, <code>42</code> as <code>XLII</code>, <code>43</code> as <code>XLIII</code>, and then <code>44</code> as <code>XLIV</code> (&#8220;<code>10</code> less than <code>50</code>, then <code>1</code> less than <code>5</code>&#8221;).
 <li>Sometimes characters are&hellip; the opposite of additive. By putting certain characters before others, you subtract from the final value. For example, at <code>9</code>, you need to subtract from the next highest tens character: <code>8</code> is <code>VIII</code>, but <code>9</code> is <code>IX</code> (&#8220;<code>1</code> less than <code>10</code>&#8221;), not <code>VIIII</code> (since the <code>I</code> character can not be repeated four times). <code>90</code> is <code>XC</code>, <code>900</code> is <code>CM</code>.
diff --git a/util/validate.py b/util/validate.py
new file mode 100644
index 0000000..403c063
--- /dev/null
+++ b/util/validate.py
@@ -0,0 +1,17 @@
+import sys
+try:
+    import html5lib
+except ImportError:
+    sys.path.insert(0, '/Users/pilgrim/code/html5lib/python3/src/')
+    import html5lib
+
+input_filename = sys.argv[1]
+parser = html5lib.HTMLParser()
+with open(input_filename, encoding='utf-8') as stream:
+    data = stream.read()
+html5doc = parser.parse(data, encoding='utf-8')
+if parser.errors:
+    for ((line, column), errtype, params) in parser.errors:
+        print("Error: {} {} on line {} of {}".format(errtype, repr(params), line, input_filename), file=sys.stderr)
+    sys.exit(1)
+