[CSS2] add lang-xml to some pre elements

This just adds lang-xml to pre elements with no lang-* class whose
contents can be parsed as XML, as it is unlikely for arbitrary content
to parse as XML (potentially some of this is intended to be HTML, but
highlighting it as if it is XML is unlikely to be harmful).

Author: gsnedders

css2/Overview.bs

@@ -4130,7 +4130,7 @@ can often be described by a fictional tag sequence that closes and
 then re-opens the element. Thus, if we mark up the previous paragraph
 with a SPAN element:</p>
 
-<pre>
+<pre class="lang-xml">
 &lt;P&gt;<b>&lt;SPAN class="test"&gt;</b> This is a somewhat long HTML
 paragraph that will be broken into several
 lines.<b>&lt;/SPAN&gt;</b> The first line will be identified
@@ -4161,7 +4161,7 @@ container element.</a>
 element may occur inside a
 block-level descendant in the same flow (i.e., a block-level
 descendant that is not positioned and not a float). E.g., the first
-line of the DIV in <code>&lt;DIV&gt;&lt;P&gt;This
+line of the DIV in <code class="lang-xml">&lt;DIV&gt;&lt;P&gt;This
 line...&lt;/P&gt;&lt;/DIV&gt;</code> is the first line of the P (assuming
 that both P and DIV are block-level).
 
@@ -4182,7 +4182,7 @@ block-level element. (Since CSS1 and CSS2&nbsp;(1998) were silent on this case,
 authors should not rely on this behavior.) Here is an example. The
 fictional tag sequence for
 
-<pre>
+<pre class="lang-xml">
 &lt;DIV&gt;
   &lt;P&gt;First paragraph&lt;/P&gt;
   &lt;P&gt;Second paragraph&lt;/P&gt;
@@ -4544,7 +4544,7 @@ not.
 Suppose there is an H1 element with an emphasizing element (EM)
 inside:</p>
 
-<pre>
+<pre class="lang-xml">
 &lt;H1&gt;The headline &lt;EM&gt;is&lt;/EM&gt; important!&lt;/H1&gt;
 </pre>
 
@@ -7307,7 +7307,7 @@ set to zero, the position of the float is undefined.
 <div class="example">
 <p>This HTML fragment results in the b floating to the right.
 
-<pre>
+<pre class="lang-xml">
 &lt;P&gt;a&lt;SPAN style="float: right"&gt;b&lt;/SPAN&gt;&lt;/P&gt;
 </pre>
 
@@ -7483,7 +7483,7 @@ which the
 clearance is -1em. (Assume none of the elements have borders or
 padding):
 
-<pre class="html">
+<pre class="lang-html">
 &lt;p style="<b>margin-bottom: 4em</b>"&gt;
   First paragraph.
 
@@ -10904,7 +10904,7 @@ elements that are not nested, is the following.  This shows how the
 style rules given above to number chapters and sections would apply to
 the markup given.
 
-<pre>
+<pre class="lang-xml">
                      &lt;!--"chapter" counter|"section" counter --&gt;
 &lt;body&gt;               &lt;!-- {chapter=0      |                  --&gt;
   &lt;h1&gt;About CSS&lt;/h1&gt; &lt;!--  chapter++ (=1) | {section=0       --&gt;
@@ -13444,7 +13444,7 @@ In the following style sheet and document fragment:
 anonymous inline element that surrounds the span element, causing 
 the text "Help, help!" to be blue, with the blue underlining from 
 the anonymous inline underneath it, the color being taken from the 
-blockquote element. The <code>&lt;em&gt;text&lt;/em&gt;</code> 
+blockquote element. The <code class="lang-xml">&lt;em&gt;text&lt;/em&gt;</code> 
 in the em block is also underlined,
 as it is in an in-flow block to which the underline is propagated. The final line of text is fuchsia, but the underline 
 underneath it is still the blue underline from the anonymous inline 
@@ -13749,7 +13749,7 @@ CSS&nbsp;2 does not fully define where line breaking opportunities occur.</em>
 <p>
 Given the following markup fragment, taking special note of spaces (with varied backgrounds and borders for emphasis and identification):
 </p>
-<pre><code>
+<pre class="lang-xml"><code>
 
      &lt;ltr&gt;A<span class="egbidiwsaA">&nbsp;</span>&lt;rtl&gt;<span class="egbidiwsbB">&nbsp;</span>B<span class="egbidiwsaB">&nbsp;</span>&lt;/rtl&gt;<span class="egbidiwsbC">&nbsp;</span>C&lt;/ltr&gt;
 
@@ -14404,7 +14404,7 @@ anonymous cell object to be created as well.</em>
 <p>Here is an example illustrating rule 5. The following illegal
   (X)HTML snippet defines conflicting cells:
 
-<pre>
+<pre class="lang-xml">
 &lt;table&gt;
 &lt;tr&gt;&lt;td&gt;1 &lt;/td&gt;&lt;td rowspan="2"&gt;2 &lt;/td&gt;&lt;td&gt;3 &lt;/td&gt;&lt;td&gt;4 &lt;/td&gt;&lt;/tr&gt;
 &lt;tr&gt;&lt;td colspan="2"&gt;5 &lt;/td&gt;&lt;/tr&gt;
@@ -16610,7 +16610,7 @@ category and subtotal. Some cells define marks along an axis while
 others give money spent at points within this space. The markup
 for this table is:</p>
 
-<pre>
+<pre class="lang-xml">
 &lt;TABLE&gt;
 &lt;CAPTION&gt;Travel Expense Report&lt;/CAPTION&gt;
 &lt;TR&gt;