diff --git a/htmltools/_core.py b/htmltools/_core.py
index 78ca86d..0139f8d 100644
--- a/htmltools/_core.py
+++ b/htmltools/_core.py
@@ -46,6 +46,8 @@
     _html_escape,  # type: ignore
     _flatten,  # type: ignore
     hash_deterministic,
+    MISSING,
+    MISSING_TYPE,
 )
 
 __all__ = (
@@ -194,7 +196,11 @@ def tagify(self) -> "TagList":
         return cp
 
     def save_html(
-        self, file: str, *, libdir: Optional[str] = "lib", include_version: bool = True
+        self,
+        file: str,
+        *,
+        libdir: Union[str, MISSING_TYPE] = MISSING,
+        include_version: bool = True,
     ) -> str:
         """
         Save to a HTML file.
@@ -583,7 +589,11 @@ def render(self) -> RenderedHTML:
         return {"dependencies": deps, "html": cp.get_html_string()}
 
     def save_html(
-        self, file: str, *, libdir: Optional[str] = "lib", include_version: bool = True
+        self,
+        file: str,
+        *,
+        libdir: Union[str, MISSING_TYPE] = MISSING,
+        include_version: bool = True,
     ) -> str:
         """
         Save to a HTML file.
@@ -707,7 +717,10 @@ def append(self, *args: TagChildArg) -> None:
         self._content.append(*args)
 
     def render(
-        self, *, lib_prefix: Optional[str] = "lib", include_version: bool = True
+        self,
+        *,
+        lib_prefix: Union[str, MISSING_TYPE] = MISSING,
+        include_version: bool = True,
     ) -> RenderedHTML:
         """
         Render the document.
@@ -726,7 +739,10 @@ def render(
         return rendered
 
     def save_html(
-        self, file: str, libdir: Optional[str] = "lib", include_version: bool = True
+        self,
+        file: str,
+        libdir: Union[str, MISSING_TYPE] = MISSING,
+        include_version: bool = True,
     ) -> str:
         """
         Save the document to a HTML file.
@@ -743,9 +759,10 @@ def save_html(
 
         # Directory where dependencies are copied to.
         destdir = str(Path(file).resolve().parent)
-        if libdir:
-            destdir = os.path.join(destdir, libdir)
+        libdir = resolve_missing_lib_path(libdir)
+        destdir = os.path.join(destdir, libdir)
 
+        # TODO: fix all libdir occurrences
         rendered = self.render(lib_prefix=libdir, include_version=include_version)
         for dep in rendered["dependencies"]:
             dep.copy_to(destdir, include_version=include_version)
@@ -760,12 +777,14 @@ def save_html(
     # - lib_prefix: A directoy prefix to add to <script src="[lib_prefix]/script.js">
     #   and <link rel="[lib_prefix]/style.css"> tags.
     def _gen_html_tag_tree(
-        self, lib_prefix: Optional[str], include_version: bool
+        self, lib_prefix: Union[str, MISSING_TYPE], include_version: bool
     ) -> Tag:
         content: TagList = self._content
         html: Tag
         body: Tag
 
+        lib_prefix = resolve_missing_lib_path(lib_prefix)
+
         if (
             len(content) == 1
             and isinstance(content[0], Tag)
@@ -797,7 +816,7 @@ def _gen_html_tag_tree(
     # <link> and <script> tags.
     @staticmethod
     def _hoist_head_content(
-        x: Tag, lib_prefix: Optional[str], include_version: bool
+        x: Tag, lib_prefix: Union[str, MISSING_TYPE], include_version: bool
     ) -> Tag:
         if x.name != "html":
             raise ValueError(f"Expected <html> tag, got <{x.name}>.")
@@ -973,13 +992,18 @@ def __init__(
             self.head = TagList(head)
 
     def source_path_map(
-        self, *, lib_prefix: Optional[str] = "lib", include_version: bool = True
+        self,
+        *,
+        lib_prefix: Union[str, MISSING_TYPE] = MISSING,
+        include_version: bool = True,
     ) -> SourcePathMapping:
         """
         Returns a dict of the absolute 'source' filepath and the 'href' path it will
         point to in the HTML (given the lib_prefix).
         """
 
+        lib_prefix = resolve_missing_lib_path(lib_prefix)
+
         src = self.source
         if src is None:
             return {"source": "", "href": ""}
@@ -997,7 +1021,10 @@ def source_path_map(
         return {"source": source, "href": href}
 
     def as_html_tags(
-        self, *, lib_prefix: Optional[str] = "lib", include_version: bool = True
+        self,
+        *,
+        lib_prefix: Union[str, MISSING_TYPE] = MISSING,
+        include_version: bool = True,
     ) -> TagList:
         """
         Render the dependency as a ``TagList()``.
@@ -1009,7 +1036,10 @@ def as_html_tags(
         return TagList(*metas, *links, *scripts, self.head)
 
     def as_dict(
-        self, *, lib_prefix: Optional[str] = "lib", include_version: bool = True
+        self,
+        *,
+        lib_prefix: Union[str, MISSING_TYPE] = MISSING,
+        include_version: bool = True,
     ) -> Dict[str, Any]:
         """
         Returns a dict of the dependency's attributes.
@@ -1054,7 +1084,7 @@ def copy_to(self, path: str, include_version: bool = True) -> None:
         Copy the dependency's files to the given path.
         """
 
-        paths = self.source_path_map(lib_prefix=None, include_version=include_version)
+        paths = self.source_path_map(lib_prefix="", include_version=include_version)
         if paths["source"] == "":
             return None
 
@@ -1245,3 +1275,10 @@ def _equals_impl(x: Any, y: Any) -> bool:
         if getattr(x, key, None) != getattr(y, key, None):
             return False
     return True
+
+
+def resolve_missing_lib_path(lib: Union[str, MISSING_TYPE] = MISSING) -> str:
+    if isinstance(lib, MISSING_TYPE):
+        return os.getenv("HTMLTOOLS_LIB_PREFIX", "lib")
+    else:
+        return lib
diff --git a/htmltools/_util.py b/htmltools/_util.py
index 1569bd9..345d1df 100644
--- a/htmltools/_util.py
+++ b/htmltools/_util.py
@@ -132,7 +132,7 @@ def hash_deterministic(s: str) -> str:
     """
     Returns a deterministic hash of the given string.
     """
-    return hashlib.sha1(s.encode('utf-8')).hexdigest()
+    return hashlib.sha1(s.encode("utf-8")).hexdigest()
 
 
 class _HttpServerInfo(NamedTuple):
@@ -175,3 +175,11 @@ def get_open_port() -> int:
     with closing(socket()) as sock:
         sock.bind(("", 0))
         return sock.getsockname()[1]
+
+
+# Sentinel value - indicates a missing value in a function call.
+class MISSING_TYPE:
+    pass
+
+
+MISSING = MISSING_TYPE()
diff --git a/tests/test_deps.py b/tests/test_deps.py
index 7ad7d61..3c0ad6a 100644
--- a/tests/test_deps.py
+++ b/tests/test_deps.py
@@ -15,7 +15,7 @@ def test_dep_resolution():
     )
     c1_0 = HTMLDependency("c", "1.0", source={"subdir": "foo"}, script={"src": "c1.js"})
     test = TagList(a1_1, b1_9, b1_10, a1_2, a1_2_1, b1_9, b1_10, c1_0)
-    assert HTMLDocument(test).render(lib_prefix=None)["html"] == textwrap.dedent(
+    assert HTMLDocument(test).render(lib_prefix="")["html"] == textwrap.dedent(
         """\
         <!DOCTYPE html>
         <html>
@@ -57,9 +57,7 @@ def test_inline_deps(snapshot):
         TagList([a1_1, div("foo")], "bar"),
         div([a1_1, div("foo")], "bar"),
     ]
-    html_ = "\n\n".join(
-        [HTMLDocument(t).render(lib_prefix=None)["html"] for t in tests]
-    )
+    html_ = "\n\n".join([HTMLDocument(t).render(lib_prefix="")["html"] for t in tests])
     snapshot.assert_match(html_, "inline_deps")
 
 
@@ -86,16 +84,16 @@ def test_append_deps():
 
     x = div(a1_1, b1_0)
     x.append(a1_2)
-    assert HTMLDocument(x).render(lib_prefix=None)["html"] == expected_result
+    assert HTMLDocument(x).render(lib_prefix="")["html"] == expected_result
 
     y = div(a1_1)
     y.append([a1_2, b1_0])
-    assert HTMLDocument(y).render(lib_prefix=None)["html"] == expected_result
+    assert HTMLDocument(y).render(lib_prefix="")["html"] == expected_result
 
     z = div()
     z.append([a1_1, b1_0])
     z.append(a1_2)
-    assert HTMLDocument(z).render(lib_prefix=None)["html"] == expected_result
+    assert HTMLDocument(z).render(lib_prefix="")["html"] == expected_result
 
 
 def test_script_input():
@@ -213,7 +211,7 @@ def test_as_dict():
         stylesheet=[{"href": "b1.css"}, {"href": "b2.css"}],
         head=tags.script("1 && 1"),
     )
-    assert b.as_dict(lib_prefix=None) == {
+    assert b.as_dict(lib_prefix="") == {
         "name": "b",
         "version": "2.0",
         "script": [],
diff --git a/tests/test_html_document.py b/tests/test_html_document.py
index fc58047..f6df080 100644
--- a/tests/test_html_document.py
+++ b/tests/test_html_document.py
@@ -140,7 +140,7 @@ def tagify(self):
     result = x.tagify().render()
     assert result["dependencies"] == [DelayedDep.dep]
 
-    result = HTMLDocument(x).render(lib_prefix=None)
+    result = HTMLDocument(x).render(lib_prefix="")
     assert result["dependencies"] == [DelayedDep.dep]
     assert result["html"].find('<script src="testdep-1.0/testdep.js">') != -1
     assert (
diff --git a/tests/test_tags.py b/tests/test_tags.py
index ed258e0..d5d5efa 100644
--- a/tests/test_tags.py
+++ b/tests/test_tags.py
@@ -229,7 +229,7 @@ def test_html_save():
         stylesheet={"href": "testdep/testdep.css"},
         script={"src": "testdep/testdep.js"},
     )
-    assert saved_html(div("foo", dep), libdir=None) == textwrap.dedent(
+    assert saved_html(div("foo", dep), libdir="") == textwrap.dedent(
         """\
         <!DOCTYPE html>
         <html>
