indent <ul> inside <ol> three spaces instead of two

ChangeLog.rst

@@ -5,6 +5,7 @@ UNRELEASED
 * Feature #318: Make padded tables more similar to pandoc's pipe_tables.
 * Add support for Python 3.9.
 * Fix extra line breaks inside html link text (between '[' and ']')
+* Fix #344: indent ``<ul>`` inside ``<ol>`` three spaces instead of two to comply with CommonMark, GFM, etc.
 
 2020.1.16
 =========

html2text/__init__.py

@@ -611,11 +611,19 @@ def link_url(self: HTML2Text, link: str, title: str = "") -> None:
                 else:
                     li = ListElement("ul", 0)
                 if self.google_doc:
-                    nest_count = self.google_nest_count(tag_style)
+                    self.o("  " * self.google_nest_count(tag_style))
                 else:
-                    nest_count = len(self.list)
-                # TODO: line up <ol><li>s > 9 correctly.
-                self.o("  " * nest_count)
+                    # Indent two spaces per list, except use three spaces for an
+                    # unordered list inside an ordered list.
+                    # https://spec.commonmark.org/0.28/#motivation
+                    # TODO: line up <ol><li>s > 9 correctly.
+                    parent_list = None
+                    for list in self.list:
+                        self.o(
+                            "   " if parent_list == "ol" and list.name == "ul" else "  "
+                        )
+                        parent_list = list.name
+
                 if li.name == "ul":
                     self.o(self.ul_item_mark + " ")
                 elif li.name == "ol":

test/mixed_nested_lists.html

@@ -0,0 +1,20 @@
+<ol>
+  <li>ordered</li>
+  <li>...</li>
+  <ul>
+    <li>unordered</li>
+    <li>...</li>
+  </ul>
+  <li>end</li>
+</ol>
+
+<ul>
+  <li>unordered</li>
+  <li>...</li>
+  <ol>
+    <li>ordered</li>
+    <li>...</li>
+  </ol>
+  <li>end</li>
+</ul>
+

test/mixed_nested_lists.md

@@ -0,0 +1,12 @@
+  1. ordered
+  2. ...
+     * unordered
+     * ...
+  3. end
+
+  * unordered
+  * ...
+    1. ordered
+    2. ...
+  * end
+