diff --git a/pkgs/markdown/CHANGELOG.md b/pkgs/markdown/CHANGELOG.md
index d7d580c..47a307c 100644
--- a/pkgs/markdown/CHANGELOG.md
+++ b/pkgs/markdown/CHANGELOG.md
@@ -1,3 +1,10 @@
+## 0.9.0
+
+* Formalize an API for Markdown extensions (#43).
+* **Breaking:** Fenced code blocks are now considered an extension, as
+  they are not part of Markdown.pl.
+* Inline HTML syntax supported. This is also considered an extension (#18).
+
 ## 0.8.0
 
 * **Breaking:** Remove (probably unused) fields: `LinkSyntax.resolved`,
diff --git a/pkgs/markdown/README.md b/pkgs/markdown/README.md
index 6237377..0cbd748 100644
--- a/pkgs/markdown/README.md
+++ b/pkgs/markdown/README.md
@@ -1,5 +1,5 @@
-A portable markdown library written in Dart. It can parse markdown into
-html on both the client and server.
+A portable Markdown library written in Dart. It can parse Markdown into
+HTML on both the client and server.
 
 Usage
 -----
@@ -13,6 +13,37 @@
 }
 ```
 
+Syntax extensions
+-----------------
+
+A few Markdown extensions are supported. They are all disabled by default, and
+can be enabled by specifying an Array of extension syntaxes in the `blockSyntaxes` or `inlineSyntaxes`
+argument of `markdownToHtml`.
+
+The currently supported inline extension syntaxes are:
+
+* `new InlineHtmlSyntax()` - approximately CommonMark's
+  [definition](http://spec.commonmark.org/0.22/#raw-html) of "Raw HTML".
+
+The currently supported block extension syntaxes are:
+
+* `const FencedCodeBlockSyntax()` - Code blocks familiar to Pandoc and PHP
+  Markdown Extra users.
+
+For example:
+
+```dart
+import 'package:markdown/markdown.dart';
+
+void main() {
+  print(markdownToHtml('Hello <span class="green">Markdown</span>',
+      inlineSyntaxes: [new InlineHtmlSyntax()]));
+  //=> <p>Hello <span class="green">Markdown</span></p>
+}
+```
+
+### Custom syntax extensions
+
 You can create and use your own syntaxes.
 
 ```dart
diff --git a/pkgs/markdown/lib/src/block_parser.dart b/pkgs/markdown/lib/src/block_parser.dart
index 67e4bbd..13c5b68 100644
--- a/pkgs/markdown/lib/src/block_parser.dart
+++ b/pkgs/markdown/lib/src/block_parser.dart
@@ -55,10 +55,31 @@
   /// The markdown document this parser is parsing.
   final Document document;
 
-  /// Index of the current line.
-  int _pos;
+  /// The enabled block syntaxes. To turn a series of lines into blocks, each of
+  /// these will be tried in turn. Order matters here.
+  final List<BlockSyntax> blockSyntaxes = [];
 
-  BlockParser(this.lines, this.document) : _pos = 0;
+  /// Index of the current line.
+  int _pos = 0;
+
+  /// The collection of built-in block parsers.
+  final List<BlockSyntax> standardBlockSyntaxes = const [
+    const EmptyBlockSyntax(),
+    const BlockHtmlSyntax(),
+    const SetextHeaderSyntax(),
+    const HeaderSyntax(),
+    const CodeBlockSyntax(),
+    const BlockquoteSyntax(),
+    const HorizontalRuleSyntax(),
+    const UnorderedListSyntax(),
+    const OrderedListSyntax(),
+    const ParagraphSyntax()
+  ];
+
+  BlockParser(this.lines, this.document) {
+    blockSyntaxes.addAll(document.blockSyntaxes);
+    blockSyntaxes.addAll(standardBlockSyntaxes);
+  }
 
   /// Gets the current line.
   String get current => lines[_pos];
@@ -90,21 +111,6 @@
 }
 
 abstract class BlockSyntax {
-  /// Gets the collection of built-in block parsers. To turn a series of lines
-  /// into blocks, each of these will be tried in turn. Order matters here.
-  static const List<BlockSyntax> syntaxes = const [
-    const EmptyBlockSyntax(),
-    const BlockHtmlSyntax(),
-    const SetextHeaderSyntax(),
-    const HeaderSyntax(),
-    const CodeBlockSyntax(),
-    const FencedCodeBlockSyntax(),
-    const BlockquoteSyntax(),
-    const HorizontalRuleSyntax(),
-    const UnorderedListSyntax(),
-    const OrderedListSyntax(),
-    const ParagraphSyntax()
-  ];
 
   const BlockSyntax();
 
@@ -136,7 +142,7 @@
   /// Gets whether or not [parser]'s current line should end the previous block.
   static bool isAtBlockEnd(BlockParser parser) {
     if (parser.isDone) return true;
-    return syntaxes.any((s) => s.canParse(parser) && s.canEndBlock);
+    return parser.blockSyntaxes.any((s) => s.canParse(parser) && s.canEndBlock);
   }
 }
 
diff --git a/pkgs/markdown/lib/src/document.dart b/pkgs/markdown/lib/src/document.dart
index 71a29d9..1190b96 100644
--- a/pkgs/markdown/lib/src/document.dart
+++ b/pkgs/markdown/lib/src/document.dart
@@ -4,14 +4,19 @@
 import 'block_parser.dart';
 import 'inline_parser.dart';
 
-/// Maintains the context needed to parse a markdown document.
+/// Maintains the context needed to parse a Markdown document.
 class Document {
   final Map<String, Link> refLinks;
+  List<BlockSyntax> blockSyntaxes;
   List<InlineSyntax> inlineSyntaxes;
   Resolver linkResolver;
   Resolver imageLinkResolver;
 
-  Document({this.inlineSyntaxes, this.linkResolver, this.imageLinkResolver})
+  Document(
+      {this.blockSyntaxes: const [],
+      this.inlineSyntaxes: const [],
+      this.linkResolver,
+      this.imageLinkResolver})
       : refLinks = <String, Link>{};
 
   parseRefLinks(List<String> lines) {
@@ -61,7 +66,7 @@
 
     var blocks = <Node>[];
     while (!parser.isDone) {
-      for (var syntax in BlockSyntax.syntaxes) {
+      for (var syntax in parser.blockSyntaxes) {
         if (syntax.canParse(parser)) {
           var block = syntax.parse(parser);
           if (block != null) blocks.add(block);
diff --git a/pkgs/markdown/lib/src/html_renderer.dart b/pkgs/markdown/lib/src/html_renderer.dart
index 87fb05c..e477721 100644
--- a/pkgs/markdown/lib/src/html_renderer.dart
+++ b/pkgs/markdown/lib/src/html_renderer.dart
@@ -10,11 +10,13 @@
 
 /// Converts the given string of markdown to HTML.
 String markdownToHtml(String markdown,
-    {List<InlineSyntax> inlineSyntaxes,
+    {List<BlockSyntax> blockSyntaxes: const [],
+    List<InlineSyntax> inlineSyntaxes: const [],
     Resolver linkResolver,
     Resolver imageLinkResolver,
     bool inlineOnly: false}) {
   var document = new Document(
+      blockSyntaxes: blockSyntaxes,
       inlineSyntaxes: inlineSyntaxes,
       imageLinkResolver: imageLinkResolver,
       linkResolver: linkResolver);
diff --git a/pkgs/markdown/lib/src/inline_parser.dart b/pkgs/markdown/lib/src/inline_parser.dart
index 9f0db08..c549067 100644
--- a/pkgs/markdown/lib/src/inline_parser.dart
+++ b/pkgs/markdown/lib/src/inline_parser.dart
@@ -70,13 +70,10 @@
 
   InlineParser(this.source, this.document) : _stack = <TagState>[] {
     // User specified syntaxes are the first syntaxes to be evaluated.
-    if (document.inlineSyntaxes != null) {
-      syntaxes.addAll(document.inlineSyntaxes);
-    }
-
+    syntaxes.addAll(document.inlineSyntaxes);
     syntaxes.addAll(_defaultSyntaxes);
 
-    // Custom link resolvers goes after the generic text syntax.
+    // Custom link resolvers go after the generic text syntax.
     syntaxes.insertAll(1, [
       new LinkSyntax(linkResolver: document.linkResolver),
       new ImageLinkSyntax(linkResolver: document.imageLinkResolver)
@@ -202,6 +199,19 @@
   }
 }
 
+/// Leave inline HTML tags alone, from
+/// [CommonMark 0.22](http://spec.commonmark.org/0.22/#raw-html).
+///
+/// This is not actually a good definition (nor CommonMark's) of an HTML tag,
+/// but it is fast. It will leave text like <a href='hi"> alone, which is
+/// incorrect.
+///
+/// TODO(srawlins): improve accuracy while ensuring performance, once
+/// Markdown benchmarking is more mature.
+class InlineHtmlSyntax extends TextSyntax {
+  InlineHtmlSyntax() : super(r'</?[A-Za-z][^>]*>');
+}
+
 /// Matches autolinks like `<http://foo.com>`.
 class AutolinkSyntax extends InlineSyntax {
   AutolinkSyntax() : super(r'<((http|https|ftp)://[^>]*)>');
diff --git a/pkgs/markdown/pubspec.yaml b/pkgs/markdown/pubspec.yaml
index e459456..9525f32 100644
--- a/pkgs/markdown/pubspec.yaml
+++ b/pkgs/markdown/pubspec.yaml
@@ -1,5 +1,5 @@
 name: markdown
-version: 0.8.0
+version: 0.9.0-dev
 author: Dart Team <misc@dartlang.org>
 description: A library for converting markdown to HTML.
 homepage: https://github.com/dart-lang/markdown
diff --git a/pkgs/markdown/test/original/fenced_code_blocks.unit b/pkgs/markdown/test/extensions/fenced_code_blocks.unit
similarity index 100%
rename from pkgs/markdown/test/original/fenced_code_blocks.unit
rename to pkgs/markdown/test/extensions/fenced_code_blocks.unit
diff --git a/pkgs/markdown/test/extensions/inline_html.unit b/pkgs/markdown/test/extensions/inline_html.unit
new file mode 100644
index 0000000..4aa260b
--- /dev/null
+++ b/pkgs/markdown/test/extensions/inline_html.unit
@@ -0,0 +1,17 @@
+>>> within a paragraph
+Within a <em class="x">paragraph</EM>.
+
+<<<
+<p>Within a <em class="x">paragraph</EM>.</p>
+>>> not HTML
+Obviously, 3 < 5 and 7 > 2.
+Not HTML: <3>, <_a>, <>
+
+<<<
+<p>Obviously, 3 &lt; 5 and 7 > 2.
+Not HTML: &lt;3>, &lt;_a>, &lt;></p>
+>>> "markdown" within a tag is not parsed
+Text <a href="_foo_">And "_foo_"</a>.
+
+<<<
+<p>Text <a href="_foo_">And "<em>foo</em>"</a>.</p>
diff --git a/pkgs/markdown/test/markdown_test.dart b/pkgs/markdown/test/markdown_test.dart
index c2804c4..8162f0d 100644
--- a/pkgs/markdown/test/markdown_test.dart
+++ b/pkgs/markdown/test/markdown_test.dart
@@ -99,4 +99,10 @@
         1. This will not be an &lt;ol>.
         ''', inlineOnly: true);
   });
+
+  testFile('extensions/fenced_code_blocks.unit',
+      blockSyntaxes: [const FencedCodeBlockSyntax()]);
+
+  testFile('extensions/inline_html.unit',
+      inlineSyntaxes: [new InlineHtmlSyntax()]);
 }
diff --git a/pkgs/markdown/test/util.dart b/pkgs/markdown/test/util.dart
index 2680a28..22d1935 100644
--- a/pkgs/markdown/test/util.dart
+++ b/pkgs/markdown/test/util.dart
@@ -15,60 +15,90 @@
 
 /// Run tests defined in "*.unit" files inside directory [name].
 void testDirectory(String name) {
-  // Locate the "test" directory. Use mirrors so that this works with the test
-  // package, which loads this suite into an isolate.
-  var testDir = p.dirname(currentMirrorSystem()
-      .findLibrary(#markdown.test.util)
-      .uri
-      .path);
-
-  var dir = p.join(testDir, name);
+  var dir = p.join(_testDir, name);
   var entries =
       new Directory(dir).listSync().where((e) => e.path.endsWith('.unit'));
 
   for (var entry in entries) {
-    group("$name ${p.basename(entry.path)}", () {
-      var lines = (entry as File).readAsLinesSync();
-
-      var i = 0;
-      while (i < lines.length) {
-        var description = lines[i++].replaceAll(">>>", "").trim();
-
-        // Let the test specify a leading indentation. This is handy for
-        // regression tests which often come from a chunk of nested code.
-        var indentMatch = _indentPattern.firstMatch(description);
-        if (indentMatch != null) {
-          // The test specifies it in spaces, but the formatter expects levels.
-          description = description.substring(indentMatch.end);
-        }
-
-        if (description == "") {
-          description = "line ${i + 1}";
-        } else {
-          description = "line ${i + 1}: $description";
-        }
-
-        var input = "";
-        while (!lines[i].startsWith("<<<")) {
-          input += lines[i++] + "\n";
-        }
-
-        var expectedOutput = "";
-        while (++i < lines.length && !lines[i].startsWith(">>>")) {
-          expectedOutput += lines[i] + "\n";
-        }
-
-        validateCore(description, input, expectedOutput);
-      }
-    });
+    testUnitFile(name, entry);
   }
 }
 
-void validateCore(String description, String markdown, String html,
-    {List<InlineSyntax> inlineSyntaxes, Resolver linkResolver,
-    Resolver imageLinkResolver, bool inlineOnly: false}) {
+// Locate the "test" directory. Use mirrors so that this works with the test
+// package, which loads this suite into an isolate.
+String get _testDir => p.dirname(currentMirrorSystem()
+      .findLibrary(#markdown.test.util)
+      .uri
+      .path);
+
+void testFile(String file,
+    {List<BlockSyntax> blockSyntaxes: const [],
+    List<InlineSyntax> inlineSyntaxes: const []}) =>
+  testUnitFile(
+      file,
+      new File(p.join(_testDir, file)),
+      blockSyntaxes: blockSyntaxes,
+      inlineSyntaxes: inlineSyntaxes);
+
+void testUnitFile(
+    String directory,
+    File entry,
+    {List<BlockSyntax> blockSyntaxes: const [],
+    List<InlineSyntax> inlineSyntaxes: const []}) {
+  group('$directory ${p.basename(entry.path)}', () {
+    var lines = entry.readAsLinesSync();
+
+    var i = 0;
+    while (i < lines.length) {
+      var description = lines[i++].replaceAll(">>>", "").trim();
+
+      // Let the test specify a leading indentation. This is handy for
+      // regression tests which often come from a chunk of nested code.
+      var indentMatch = _indentPattern.firstMatch(description);
+      if (indentMatch != null) {
+        // The test specifies it in spaces, but the formatter expects levels.
+        description = description.substring(indentMatch.end);
+      }
+
+      if (description == "") {
+        description = "line ${i + 1}";
+      } else {
+        description = "line ${i + 1}: $description";
+      }
+
+      var input = "";
+      while (!lines[i].startsWith("<<<")) {
+        input += lines[i++] + "\n";
+      }
+
+      var expectedOutput = "";
+      while (++i < lines.length && !lines[i].startsWith(">>>")) {
+        expectedOutput += lines[i] + "\n";
+      }
+
+      validateCore(
+          description,
+          input,
+          expectedOutput,
+          blockSyntaxes: blockSyntaxes,
+          inlineSyntaxes: inlineSyntaxes
+          );
+    }
+  });
+}
+
+void validateCore(
+    String description,
+    String markdown,
+    String html,
+    {List<BlockSyntax> blockSyntaxes: const [],
+    List<InlineSyntax> inlineSyntaxes: const [],
+    Resolver linkResolver,
+    Resolver imageLinkResolver,
+    bool inlineOnly: false}) {
   test(description, () {
     var result = markdownToHtml(markdown,
+        blockSyntaxes: blockSyntaxes,
         inlineSyntaxes: inlineSyntaxes,
         linkResolver: linkResolver,
         imageLinkResolver: imageLinkResolver,