diff --git a/docs/Makefile b/docs/Makefile
index 57d350d..c3b5db9 100644
--- a/docs/Makefile
+++ b/docs/Makefile
@@ -9,7 +9,7 @@ SPHINXBUILD   = python3 -msphinx
 SOURCEDIR     = .
 BUILDDIR      = _build
 
-default: html
+default: fasthtml
 
 # Put it first so that "make" without argument is like "make help".
 help:
@@ -17,7 +17,14 @@ help:
 
 .PHONY: help Makefile default
 
-# Catch-all target: route all unknown targets to Sphinx using the new
-# "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
+# Catch-all target: route all unknown targets to Sphinx builder.
+# $(O) is meant as a shortcut for $(SPHINXOPTS).
+#
+# NOTE: We want the html builder and fasthtml builder share same outdir.
+#
+# 	1. Don't use the make mode (-M) because it forces $(BUILDDIR)/$(BUILDERNAME)
+# 		as outdir
+# 	2. The $(if ...) expr returns "html" as outdir for when fasthtml builder is
+# 		specified, otherwise return the original builder name
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@$(SPHINXBUILD) -b $@ "$(SOURCEDIR)" "$(BUILDDIR)/$(if $@=fasthtml,html,$@)" $(SPHINXOPTS) $(O) 
diff --git a/src/sphinxnotes/fasthtml/__init__.py b/src/sphinxnotes/fasthtml/__init__.py
new file mode 100644
index 0000000..38c8d45
--- /dev/null
+++ b/src/sphinxnotes/fasthtml/__init__.py
@@ -0,0 +1,107 @@
+"""
+    sphinxnotes.fasthtml
+    ~~~~~~~~~~~~~~~~~~~~
+
+    Sphinx builder specialized for fast incremental HTML build 
+
+    :copyright: Copyright 2024 Shengyu Zhang
+    :license: BSD, see LICENSE for details.
+
+TODO:
+
+- [ ] skip 'checking consistency'
+- [ ] why always [config changed ('gettext_auto_build')]?
+- [ ] config-able.
+
+"""
+
+from __future__ import annotations
+from typing import TYPE_CHECKING
+
+from sphinx.builders.html import StandaloneHTMLBuilder
+from sphinx.util import logging
+from sphinx.environment import CONFIG_OK
+
+if TYPE_CHECKING:
+    from sphinx.application import Sphinx
+    from sphinx.environment import BuildEnvironment
+
+logger = logging.getLogger(__name__)
+
+class FastHTMLBuilder(StandaloneHTMLBuilder):
+    name = 'fasthtml'
+
+    def __init__(self, app: Sphinx, env: BuildEnvironment) -> None:
+        # We only use the 'fasthtml' for registering builder,
+        # then continue to use the same name as StandaloneHTMLBuilder,
+        # to ensure that the behavior with StandaloneHTMLBuilderi as same
+        # as possible.
+        #
+        # Otherwise, different builde name causes troubles: 
+        #
+        # - Builder.tags will be different (see Builder.init()), which leads
+        #   to BuildInfo changes, finally leads Builder.get_outdated_docs()
+        #   returns all docs and do a full rebuild.
+        # - sphinxnotes-any (another project of mine) creates a directory for
+        #   storing intermediate files according to builder name (.any_xxx/),
+        #   if builder names are different, intermediate files can't be shared
+        #   between {Fast,Standalone}HTMLBuilder, which leas to unnecessary
+        #   rebuild.
+        self.name = StandaloneHTMLBuilder.name
+        super().__init__(app, env)
+
+
+    def gen_pages_from_extensions(self) -> None:
+        pass # skip gen
+
+
+def _on_builder_inited(app: Sphinx):
+    if not isinstance(app.builder, FastHTMLBuilder):
+        return
+
+    # Disable general index.
+    app.config.html_use_index = False # type: ignore
+    app.builder.use_index = False
+    # Disable domain-specific indices.
+    app.config.html_domain_indices = False # type: ignore
+
+    # Disable search.
+    app.builder.search = False
+
+    # Do not update toctree.
+    app.env.glob_toctrees = set()
+    app.env.reread_always = set() # marked by env.note_reread()
+
+    # Do not build mo files.
+    app.config.gettext_auto_build = False # type: ignore
+
+
+def _on_env_get_outdated(app: Sphinx, env: BuildEnvironment, added: set[str],
+                         changed: set[str], removed: set[str]) -> list[str]:
+    if not isinstance(app.builder, FastHTMLBuilder):
+        return []
+
+    # Config changes causes a fully rebuild, I don't want this.
+    if env.config_status != CONFIG_OK:
+        # Require the env to recalculate which docs should be rebuilt when the
+        # configuration has *NOT* changed.
+        added2, changed2, removed2 = env.get_outdated_files(config_changed=False)
+
+        def clear_and_update(dst, src):
+            dst.clear(); dst.update(src)
+        # sphinx.builders.Builder.read [#]_ saids "allow user intervention" when
+        # emitting "env-get-outdated" signal. My understanding is that it allows
+        # us to modify the docnames sets set passed in.
+        #
+        # .. [#]: https://github.com/sphinx-doc/sphinx/blob/v7.3.7/sphinx/builders/__init__.py#L382
+        clear_and_update(added, added2)
+        clear_and_update(changed, changed2)
+        clear_and_update(removed, removed2)
+
+    return []
+
+def setup(app: Sphinx):
+    app.connect('builder-inited', _on_builder_inited, priority=100)
+    app.connect('env-get-outdated', _on_env_get_outdated)
+
+    app.add_builder(FastHTMLBuilder)