fix: strip <ol>-only attrs (start, type, reversed) from <li>/<dd>

docs/reference/enhancements.md

@@ -441,6 +441,12 @@ Works with all list types:
   block attribute for that following block; the list and item are
   not terminated.
 
+**Stripped attribute names:** `start`, `type`, and `reversed` are HTML
+attributes valid only on `<ol>`. When authored on a list item they are
+silently dropped from the rendered `<li>` to keep output HTML valid.
+To set `<ol start="5">`, use a standard block-attribute line **before**
+the list: `{start=5}` followed by `1. ...`.
+
 ---
 
 ### Table Row and Cell Attributes
@@ -749,6 +755,9 @@ Attributes can be attached to individual `<dl>`, `<dt>`, and `<dd>` elements:
 - `{...}` on line after term → applies to that `<dt>`
 - `{...}` as last line in definition block → applies to that `<dd>` (consistent with list items)
 
+**Stripped attribute names:** the same `start`, `type`, `reversed`
+strip applies to `<dd>` output.
+
 ---
 
 ### Table Multi-line Cells, Rowspan, and Colspan

src/Renderer/HtmlRenderer.php

@@ -88,6 +88,15 @@ class HtmlRenderer implements RendererInterface
      */
     protected array $nodeRenderers = [];
 
+    /**
+     * Attribute names valid on <ol> but not on <li>/<dd>. Stripped from
+     * <li>/<dd> output to avoid emitting invalid HTML when authors put
+     * these names on a list item (e.g. as a postfix {start=5} line).
+     *
+     * @var array<int, string>
+     */
+    protected const OL_ONLY_ATTRIBUTES = ['start', 'type', 'reversed'];
+
     public function __construct(protected bool $xhtml = false)
     {
         $this->sharedRenderContext = new RenderContext();
@@ -741,7 +750,7 @@ protected function renderList(ListBlock $node): string
 
     protected function renderListItem(ListItem $node, bool $tight = true): string
     {
-        $attrs = $this->renderAttributes($node);
+        $attrs = $this->renderAttributesExcluding($node, self::OL_ONLY_ATTRIBUTES);
         $content = $this->renderChildren($node);
 
         if ($tight) {
@@ -1239,7 +1248,7 @@ protected function renderDefinitionTerm(DefinitionTerm $node): string
 
     protected function renderDefinitionDescription(DefinitionDescription $node): string
     {
-        $attrs = $this->renderAttributes($node);
+        $attrs = $this->renderAttributesExcluding($node, self::OL_ONLY_ATTRIBUTES);
         $content = $this->renderChildren($node);
 
         // Content goes on separate lines

tests/TestCase/DjotConverterTest.php

@@ -1104,6 +1104,80 @@ public function testListItemAttributesLooseListUnchanged(): void
         $this->assertStringContainsString('<blockquote>', $result);
     }
 
+    public function testListItemAttributeStartIsStrippedFromLi(): void
+    {
+        // G3: `start` is an HTML attribute valid only on <ol>. It must
+        // be stripped from <li> output, never feed <ol>, and never
+        // appear in HTML.
+        $djot = "1. item\n   {start=5}\n";
+
+        $result = $this->converter->convert($djot);
+
+        $this->assertStringContainsString('<li>', $result);
+        $this->assertStringNotContainsString('<li start=', $result);
+        $this->assertStringNotContainsString('<ol start="5"', $result);
+    }
+
+    public function testListItemAttributeTypeIsStrippedFromLi(): void
+    {
+        // G3: `type` is an HTML attribute valid only on <ol>. Stripped
+        // from <li>; never overrides marker-derived <ol type=...>.
+        $djot = "(a) x\n   {type=i}\n";
+
+        $result = $this->converter->convert($djot);
+
+        $this->assertStringContainsString('<ol type="a">', $result);
+        $this->assertStringNotContainsString('<li type=', $result);
+        $this->assertStringNotContainsString('type="i"', $result);
+    }
+
+    public function testListItemAttributeReversedIsStrippedFromLi(): void
+    {
+        // G3: `reversed` is <ol>-only. Stripped from <li>.
+        $djot = "1. item\n   {reversed}\n";
+
+        $result = $this->converter->convert($djot);
+
+        $this->assertStringContainsString('<li>', $result);
+        $this->assertStringNotContainsString('<li reversed', $result);
+        $this->assertStringNotContainsString('reversed=""', $result);
+    }
+
+    public function testListItemOtherAttributesUnchanged(): void
+    {
+        // G3 regression guard: only start/type/reversed are stripped.
+        // class, id, data-* pass through unchanged on <li>.
+        $djot = "1. item\n   {#anchor .step data-step=\"1\"}\n";
+
+        $result = $this->converter->convert($djot);
+
+        $this->assertStringContainsString('id="anchor"', $result);
+        $this->assertStringContainsString('class="step"', $result);
+        $this->assertStringContainsString('data-step="1"', $result);
+    }
+
+    public function testOlStartFromBlockAttrUnchanged(): void
+    {
+        // G3 regression guard: a block-attribute line BEFORE a list
+        // applies to the <ol> and must still emit start/type as before.
+        $djot = "{start=5}\n1. item\n2. next\n";
+
+        $result = $this->converter->convert($djot);
+
+        $this->assertStringContainsString('<ol start="5"', $result);
+    }
+
+    public function testDefinitionListDdStartIsStripped(): void
+    {
+        // G3: same strip applies to <dd>.
+        $djot = ": term\n\n  definition\n  {start=5}\n";
+
+        $result = $this->converter->convert($djot);
+
+        $this->assertStringContainsString('<dd>', $result);
+        $this->assertStringNotContainsString('<dd start=', $result);
+    }
+
     public function testRomanNumeralList(): void
     {
         // x. is parsed as Roman numeral 10