diff --git a/src/librustdoc/html/render/mod.rs b/src/librustdoc/html/render/mod.rs
index a5e62baca3816..0f83346a67a15 100644
--- a/src/librustdoc/html/render/mod.rs
+++ b/src/librustdoc/html/render/mod.rs
@@ -1612,7 +1612,7 @@ fn render_impl(
                     error_codes: cx.shared.codes,
                     edition: cx.shared.edition(),
                     playground: &cx.shared.playground,
-                    heading_offset: HeadingOffset::H2
+                    heading_offset: HeadingOffset::H4
                 }
                 .into_string()
             );
diff --git a/src/librustdoc/html/render/print_item.rs b/src/librustdoc/html/render/print_item.rs
index 12ea7b4f74bce..f70369d7ab4cf 100644
--- a/src/librustdoc/html/render/print_item.rs
+++ b/src/librustdoc/html/render/print_item.rs
@@ -983,7 +983,7 @@ fn item_union(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, s: &clean::Uni
             if let Some(stability_class) = field.stability_class(cx.tcx()) {
                 write!(w, "<span class=\"stab {stab}\"></span>", stab = stability_class);
             }
-            document(w, cx, field, Some(it), HeadingOffset::H2);
+            document(w, cx, field, Some(it), HeadingOffset::H3);
         }
     }
     let def_id = it.def_id.expect_def_id();
@@ -1094,7 +1094,7 @@ fn item_enum(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, e: &clean::Enum
             w.write_str("</code>");
             render_stability_since(w, variant, it, cx.tcx());
             w.write_str("</div>");
-            document(w, cx, variant, Some(it), HeadingOffset::H2);
+            document(w, cx, variant, Some(it), HeadingOffset::H3);
             document_non_exhaustive(w, variant);
 
             use crate::clean::Variant;
@@ -1134,7 +1134,7 @@ fn item_enum(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, e: &clean::Enum
                                 f = field.name.as_ref().unwrap(),
                                 t = ty.print(cx)
                             );
-                            document(w, cx, field, Some(variant), HeadingOffset::H2);
+                            document(w, cx, field, Some(variant), HeadingOffset::H4);
                         }
                         _ => unreachable!(),
                     }
@@ -1286,7 +1286,7 @@ fn item_struct(w: &mut Buffer, cx: &Context<'_>, it: &clean::Item, s: &clean::St
                     name = field_name,
                     ty = ty.print(cx)
                 );
-                document(w, cx, field, Some(it), HeadingOffset::H2);
+                document(w, cx, field, Some(it), HeadingOffset::H3);
             }
         }
     }
diff --git a/src/test/rustdoc-gui/header-size.goml b/src/test/rustdoc-gui/header-size.goml
new file mode 100644
index 0000000000000..fa5d9c9291628
--- /dev/null
+++ b/src/test/rustdoc-gui/header-size.goml
@@ -0,0 +1,99 @@
+// This test check that headers (a) have the correct heading level, and (b) are the right size.
+// The sizes may change as design changes, but try to make sure a lower header is never bigger than
+// its parent headers.
+// Most of these sizes are set in CSS in `em` units, so here's a conversion chart based on our
+// default 16px font size:
+// 24px    1.5em
+// 22.4px  1.4em
+// 20.8px  1.3em
+// 18.4px  1.15em
+// 17.6px  1.1em
+// 16px    1em
+// 15.2px  0.95em  
+goto: file://|DOC_PATH|/test_docs/struct.HeavilyDocumentedStruct.html
+
+assert-css: ("h1.fqn", {"font-size": "24px"})
+
+assert-css: ("h2#top-doc-prose-title", {"font-size": "20.8px"})
+assert-css: ("h3#top-doc-prose-sub-heading", {"font-size": "18.4px"})
+assert-css: ("h4#top-doc-prose-sub-sub-heading", {"font-size": "17.6px"})
+
+assert-css: ("h2#fields", {"font-size": "22.4px"})
+assert-css: ("h3#title-for-field", {"font-size": "20.8px"})
+assert-css: ("h4#sub-heading-for-field", {"font-size": "16px"})
+
+assert-css: ("h2#implementations", {"font-size": "22.4px"})
+
+assert-css: ("#impl > h3.code-header", {"font-size": "17.6px"})
+assert-css: ("#method\.do_nothing > h4.code-header", {"font-size": "16px"})
+
+assert-css: ("h4#title-for-struct-impl-doc", {"font-size": "16px"})
+assert-css: ("h5#sub-heading-for-struct-impl-doc", {"font-size": "16px"})
+assert-css: ("h6#sub-sub-heading-for-struct-impl-doc", {"font-size": "15.2px"})
+
+assert-css: ("h5#title-for-struct-impl-item-doc", {"font-size": "16px"})
+assert-css: ("h6#sub-heading-for-struct-impl-item-doc", {"font-size": "15.2px"})
+assert-css: ("h6#sub-sub-heading-for-struct-impl-item-doc", {"font-size": "15.2px"})
+
+goto: file://|DOC_PATH|/test_docs/enum.HeavilyDocumentedEnum.html
+
+assert-css: ("h1.fqn", {"font-size": "24px"})
+
+assert-css: ("h2#top-doc-prose-title", {"font-size": "20.8px"})
+assert-css: ("h3#top-doc-prose-sub-heading", {"font-size": "18.4px"})
+assert-css: ("h4#top-doc-prose-sub-sub-heading", {"font-size": "17.6px"})
+
+assert-css: ("h2#variants", {"font-size": "22.4px"})
+
+assert-css: ("h3#none-prose-title", {"font-size": "20.8px"})
+assert-css: ("h4#none-prose-sub-heading", {"font-size": "16px"})
+
+assert-css: ("h3#wrapped-prose-title", {"font-size": "20.8px"})
+assert-css: ("h4#wrapped-prose-sub-heading", {"font-size": "16px"})
+
+assert-css: ("h4#wrapped0-prose-title", {"font-size": "16px"})
+assert-css: ("h5#wrapped0-prose-sub-heading", {"font-size": "16px"})
+
+assert-css: ("h4#structy-prose-title", {"font-size": "16px"})
+assert-css: ("h5#structy-prose-sub-heading", {"font-size": "16px"})
+
+assert-css: ("h2#implementations", {"font-size": "22.4px"})
+
+assert-css: ("#impl > h3.code-header", {"font-size": "17.6px"})
+assert-css: ("#method\.do_nothing > h4.code-header", {"font-size": "16px"})
+
+assert-css: ("h4#title-for-enum-impl-doc", {"font-size": "16px"})
+assert-css: ("h5#sub-heading-for-enum-impl-doc", {"font-size": "16px"})
+assert-css: ("h6#sub-sub-heading-for-enum-impl-doc", {"font-size": "15.2px"})
+
+assert-css: ("h5#title-for-enum-impl-item-doc", {"font-size": "16px"})
+assert-css: ("h6#sub-heading-for-enum-impl-item-doc", {"font-size": "15.2px"})
+assert-css: ("h6#sub-sub-heading-for-enum-impl-item-doc", {"font-size": "15.2px"})
+
+goto: file://|DOC_PATH|/test_docs/union.HeavilyDocumentedUnion.html
+
+assert-css: ("h1.fqn", {"font-size": "24px"})
+
+assert-css: ("h2#top-doc-prose-title", {"font-size": "20.8px"})
+assert-css: ("h3#top-doc-prose-sub-heading", {"font-size": "18.4px"})
+
+assert-css: ("h2#fields", {"font-size": "22.4px"})
+
+assert-css: ("h3#title-for-union-variant", {"font-size": "20.8px"})
+assert-css: ("h4#sub-heading-for-union-variant", {"font-size": "16px"})
+
+assert-css: ("h2#implementations", {"font-size": "22.4px"})
+
+assert-css: ("#impl > h3.code-header", {"font-size": "17.6px"})
+assert-css: ("h4#title-for-union-impl-doc", {"font-size": "16px"})
+assert-css: ("h5#sub-heading-for-union-impl-doc", {"font-size": "16px"})
+
+assert-css: ("h5#title-for-union-impl-item-doc", {"font-size": "16px"})
+assert-css: ("h6#sub-heading-for-union-impl-item-doc", {"font-size": "15.2px"})
+
+goto: file://|DOC_PATH|/test_docs/macro.heavily_documented_macro.html
+
+assert-css: ("h1.fqn", {"font-size": "24px"})
+
+assert-css: ("h2#top-doc-prose-title", {"font-size": "20.8px"})
+assert-css: ("h3#top-doc-prose-sub-heading", {"font-size": "18.4px"})
diff --git a/src/test/rustdoc-gui/sidebar.goml b/src/test/rustdoc-gui/sidebar.goml
index 62dc76a40bcf5..eacc9f6c15fe1 100644
--- a/src/test/rustdoc-gui/sidebar.goml
+++ b/src/test/rustdoc-gui/sidebar.goml
@@ -13,7 +13,8 @@ assert-text: (".sidebar-elems > .items > ul > li:nth-child(4)", "Enums")
 assert-text: (".sidebar-elems > .items > ul > li:nth-child(5)", "Traits")
 assert-text: (".sidebar-elems > .items > ul > li:nth-child(6)", "Functions")
 assert-text: (".sidebar-elems > .items > ul > li:nth-child(7)", "Type Definitions")
-assert-text: (".sidebar-elems > .items > ul > li:nth-child(8)", "Keywords")
+assert-text: (".sidebar-elems > .items > ul > li:nth-child(8)", "Unions")
+assert-text: (".sidebar-elems > .items > ul > li:nth-child(9)", "Keywords")
 assert-text: ("#structs + .item-table .item-left > a", "Foo")
 click: "#structs + .item-table .item-left > a"
 
diff --git a/src/test/rustdoc-gui/src/test_docs/lib.rs b/src/test/rustdoc-gui/src/test_docs/lib.rs
index 652308a71cb85..14d8b18613087 100644
--- a/src/test/rustdoc-gui/src/test_docs/lib.rs
+++ b/src/test/rustdoc-gui/src/test_docs/lib.rs
@@ -131,3 +131,129 @@ macro_rules! repro {
 }
 
 pub use crate::repro as repro2;
+
+/// # Top-doc Prose title
+///
+/// Text below title.
+///
+/// ## Top-doc Prose sub-heading
+///
+/// Text below sub-heading.
+///
+/// ### Top-doc Prose sub-sub-heading
+///
+/// Text below sub-sub-heading
+pub struct HeavilyDocumentedStruct {
+    /// # Title for field
+    /// ## Sub-heading for field
+    pub nothing: (),
+}
+
+/// # Title for struct impl doc
+///
+/// Text below heading.
+///
+/// ## Sub-heading for struct impl doc
+///
+/// Text below sub-heading.
+///
+/// ### Sub-sub-heading for struct impl doc
+///
+/// Text below sub-sub-heading.
+///
+impl HeavilyDocumentedStruct {
+    /// # Title for struct impl-item doc
+    /// Text below title.
+    /// ## Sub-heading for struct impl-item doc
+    /// Text below sub-heading.
+    /// ### Sub-sub-heading for struct impl-item doc
+    /// Text below sub-sub-heading.
+    pub fn do_nothing() {}
+}
+
+/// # Top-doc Prose title
+///
+/// Text below title.
+///
+/// ## Top-doc Prose sub-heading
+///
+/// Text below sub-heading.
+///
+/// ### Top-doc Prose sub-sub-heading
+///
+/// Text below sub-sub-heading
+pub enum HeavilyDocumentedEnum {
+    /// # None prose title
+    /// ## None prose sub-heading
+    None,
+    /// # Wrapped prose title
+    /// ## Wrapped prose sub-heading
+    Wrapped(
+        /// # Wrapped.0 prose title
+        /// ## Wrapped.0 prose sub-heading
+        String,
+        String,
+    ),
+    Structy {
+        /// # Structy prose title
+        /// ## Structy prose sub-heading
+        alpha: String,
+        beta: String,
+    },
+}
+
+/// # Title for enum impl doc
+///
+/// Text below heading.
+///
+/// ## Sub-heading for enum impl doc
+///
+/// Text below sub-heading.
+///
+/// ### Sub-sub-heading for enum impl doc
+///
+/// Text below sub-sub-heading.
+///
+impl HeavilyDocumentedEnum {
+    /// # Title for enum impl-item doc
+    /// Text below title.
+    /// ## Sub-heading for enum impl-item doc
+    /// Text below sub-heading.
+    /// ### Sub-sub-heading for enum impl-item doc
+    /// Text below sub-sub-heading.
+    pub fn do_nothing() {}
+}
+
+/// # Top-doc prose title
+///
+/// Text below heading.
+///
+/// ## Top-doc prose sub-heading
+///
+/// Text below heading.
+pub union HeavilyDocumentedUnion {
+    /// # Title for union variant
+    /// ## Sub-heading for union variant
+    pub nothing: (),
+    pub something: f32,
+}
+
+/// # Title for union impl doc
+/// ## Sub-heading for union impl doc
+impl HeavilyDocumentedUnion {
+    /// # Title for union impl-item doc
+    /// ## Sub-heading for union impl-item doc
+    pub fn do_nothing() {}
+}
+
+/// # Top-doc prose title
+///
+/// Text below heading.
+///
+/// ## Top-doc prose sub-heading
+///
+/// Text below heading.
+#[macro_export]
+macro_rules! heavily_documented_macro {
+    () => {};
+}