Merge pull request #5693 from plotly/mishrakushal-add-doctype-to_html

Add <!doctype html> to the to_html template

Author: emilykl

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 - Fix `hex_to_rgb` parsing of 3-digit shorthand hexadecimal colors such as `#FFF` [[#5662](https://github.com/plotly/plotly.py/pull/5662)], with thanks to @genrichez for the contribution!
+- Add `<!doctype html>` to the `to_html()` template to comply with modern web standards [[#5693](https://github.com/plotly/plotly.py/pull/5693)], with thanks to @mishrakushal for the contribution!
 
 
 ## [6.9.0] - 2026-07-09

plotly/io/_html.py

@@ -339,8 +339,12 @@ def to_html(
 
     if full_html:
         return """\
+<!doctype html>
 <html>
-<head><meta charset="utf-8" /></head>
+<head>
+    <meta charset="utf-8" />
+    <style>html, body {{height: 100%;}}</style>
+</head>
 <body>
     {div}
 </body>

tests/test_core/test_offline/test_offline.py

@@ -108,7 +108,7 @@ def test_default_plot_generates_expected_html(self):
         self.assertIn(plotly_config_script, html)  # so is config
         self.assertIn(PLOTLYJS, html)  # and the source code
         # and it's an <html> doc
-        self.assertTrue(html.startswith("<html>") and html.endswith("</html>"))
+        self.assertTrue(html.startswith("<!doctype html>") and html.endswith("</html>"))
 
     def test_including_plotlyjs_truthy_html(self):
         # For backwards compatibility all truthy values that aren't otherwise

tests/test_io/test_renderers.py

@@ -133,11 +133,11 @@ def test_plotly_mimetype_renderer_show(fig1, renderer):
 # HTML
 # ----
 def assert_full_html(html):
-    assert html.startswith("<html")
+    assert html.startswith("<!doctype html")
 
 
 def assert_not_full_html(html):
-    assert not html.startswith("<html")
+    assert not html.startswith("<!doctype html") and not html.startswith("<html")
 
 
 def assert_html_renderer_connected(html):

tests/test_optional/test_offline/test_offline.py

@@ -88,4 +88,6 @@ def test_default_mpl_plot_generates_expected_html(self):
             self.assertTrue(data_json in html)  # data is in there
             self.assertTrue(PLOTLYJS in html)  # and the source code
             # and it's an <html> doc
-            self.assertTrue(html.startswith("<html>") and html.endswith("</html>"))
+            self.assertTrue(
+                html.startswith("<!doctype html>") and html.endswith("</html>")
+            )