Add release notes.

docs/change_log/release-3.3.md

@@ -66,6 +66,21 @@ The following new features have been included in the 3.3 release:
   Any random HTML attribute can be defined and set on the `<code>` tag of fenced code
   blocks when the `attr_list` extension is enabled (#816).
 
+* The HTML parser has been completely replaced. The new HTML parser is built on Python's
+  [html.parser.HTMLParser](https://docs.python.org/3/library/html.parser.html), which
+  alleviates various bugs and simplify maintenance of the code (#803, #830).
+
+* The [Markdown in HTML](../md_in_html.md) extension has been rebuilt on the new HTML
+  Parser, which drastically simplifies it. Note that raw HTML elements with a `markdown`
+  attribute defined are now converted to ElementTree Elements and are rendered by the
+  serializer. Various bugs have been fixed (#803, #595, #780, and #1012).
+
+* Link reference parsing, abbreviation reference parsing and footnote reference parsing
+  has all been moved from `preprocessors` to `blockprocessors`, which allows them to be
+  nested within other block level elements. Specifically, this change was necessary to
+  maintain the current behavior in the rebuilt Markdown in HTML extension. A few random
+  edge-case bugs (see the included tests) were resolved in the process (#803).
+
 ## Bug fixes
 
 The following bug fixes are included in the 3.3 release: