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+Notes On Diff Formatting
+========================
+
+There are two main kinds of diff display for the web interface:
+unified and side-by-side.  Both displays are implemented using
+a <table>.  The unified diff is a 4-column table, and the
+side-by-side diff is a 5-column table.  In a page like /info that
+might show multiple file diffs, each file diff is in a separate
+<table>.  For side-by-side diffs, a small amount of Javascript
+code is used to resize the text columns so that they fill the screen
+width and to keep horizontal scrollbars in sync.
+
+For the unified diff, the basic structure
+is like this:
+
+> ~~~~
+<table class='diff udiff'>
+<tr>
+  <td class='diffln difflnl'><pre>
+     Line numbers for the left-hand file
+  </pre></td>
+  <td class='diffln difflnr'><pre>
+     Line numbers for the right-hand file
+  </pre></td>
+  <td class='diffsep'><pre>
+     Change marks.  "+" or "=" or nothing
+  </pre></td>
+  <td class='difftxt difftxtu'><pre>
+     The text
+  </pre></td>
+</tr>
+</table>
+~~~~
+
+The structure for a side-by-side diff follows the
+same basic pattern, though with 5 columns instead of
+4, and slightly different class names:
+
+> ~~~~
+<table class='diff splitdiff'>
+<tr>
+  <td class='diffln difflnl'><pre>
+     Line numbers for the left-hand file
+  </pre></td>
+  <td class='difftxt difftxtl'><pre>
+     The text for the left side
+  </pre></td>
+  <td class='diffsep'><pre>
+     Change marks.  "+" or "=" or nothing
+  </pre></td>
+  <td class='diffln difflnr'><pre>
+     Line numbers for the right-hand file
+  </pre></td>
+  <td class='difftxt difftxtr'><pre>
+     The text on the right-hand side
+  </pre></td>
+</tr>
+</table>
+~~~~
+
+The outer &lt;table&gt; always has class "diff".  The "diffu" class
+is added for unified diffs and the "splitdiff" class is added for
+side-by-side diffs.
+
+All line-number columns have the "diffln" class.  They also always
+have one of "difflnl" or "difflnr" depending on whether they hold
+line numbers for the left or right files, respectively.
+
+Text is always kept in a separate column so that it can be scraped
+and copied by the user.  All text columns have the "difftxt" class.
+One additional class "difftxtu", "difftxtl", or "difftxtr" is added
+depending on if the text is for a unified diff, the left column of
+a side-by-side diff, or the right column of a side-by-side diff.
+
+The content of all columns is a single &lt;pre&gt; that contains the
+appropriate diff-text for that column.  Scrolling is done on the
+&lt;pre&gt; element.
+
+Within text columns, highlighting is done with &lt;del&gt; and
+&lt;ins&gt; markup.  All text on a line that contains an isert or
+delete is surrounded by &lt;ins&gt;...&lt;/ins&gt; or
+&lt;del&gt;..&lt;/del&gt;.  Within that line, specific characters
+of text that specifically inserted deleted have an additional
+layer of &lt;ins&gt; or &lt;del&gt; markup.  Thus CSS like the
+following is appropriate:
+
+> ~~~~
+td.difftxt ins {
+  background-color: #dafbe1;  /* Light green for the whole line */
+  text-decoration: none;
+}
+td.difftxt ins > ins {
+  background-color: #a0e4b2;  /* Dark green for specific characters that change */
+  text-decoration: none;
+}
+~~~~
+
+In a side-by-side diff, if an interior &lt;ins&gt; or &lt;del&gt; that mark
+specific characters that change correspond to a delete/insert on the other
+side, they they have the "edit" class tag.  (ex:  &lt;ins&nbsp;class='edit'&gt;
+or &lt;del&nbsp;class='edit'&gt;).  Some skins choose to paint these "modified"
+regions blue:
+
+> ~~~~
+td.difftxt ins > ins.edit {
+  background-color: #c0c0ff;  /* Blue for "modified" text region */
+  text-decoration: none;
+}
+~~~~
+
+Line number text also has &lt;ins&gt; and &lt;del&gt; tags for lines which
+are pure insert or pure delete.  But the tags do not nest for line numbers.
diff --git a/www/css/index.md b/www/css/index.md
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+Cascading Style Sheet Notes
+===========================
+
+This is a collection of technical notes that document the design of
+the Document Object Model (DOM) and corresponding Cascading Style Sheet (CSS)
+attributes used for customing the look-and-feel of Fossil.  These notes
+are of interest to people who want to customize the Fossil skin or
+enhance the internal display logic.
+
+This is a collection of documents that we hope will grow over time.
+
+  *  [Diff styling](./diff.md)