fix: honor start="0" on ordered lists

The ordered-list numbering used intval() on the "start" attribute inside a
truthiness check, so a valid start="0" was treated as absent and the list
restarted numbering at 1. Detect an explicit numeric start attribute with
is_numeric() instead, so <ol start=\"0\"> renders as 0. 1. ...

PR.md

@@ -0,0 +1,73 @@
+# fix: honor start="0" on ordered lists
+
+## Problem
+
+Converting an ordered list whose `start` attribute is `0` produces the wrong
+numbering: the list restarts at `1` instead of `0`.
+
+```php
+$converter = new League\HTMLToMarkdown\HtmlConverter();
+$converter->convert('<ol start="0"><li>Item A</li><li>Item B</li></ol>');
+// actual:   "1. Item A\n2. Item B"
+// expected: "0. Item A\n1. Item B"
+```
+
+`<ol start="0">` is valid HTML, and CommonMark supports ordered lists that begin
+at `0`, so the resulting Markdown is incorrect. Any other start value (for
+example `start="120"`) works.
+
+## Root cause
+
+`src/Converter/ListItemConverter.php:55`. The start value was read inside a
+truthiness check:
+
+```php
+if ($listType === 'ol' && ($parent = $element->getParent()) && ($start = \intval($parent->getAttribute('start')))) {
+    $number = $start + $element->getSiblingPosition() - 1;
+} else {
+    $number = $element->getSiblingPosition();
+}
+```
+
+`\intval('0')` is `0`, which is falsy, so an explicit `start="0"` takes the
+`else` branch and the list is renumbered from the sibling position (1-based).
+The check cannot distinguish "start attribute absent" from "start attribute set
+to 0", because `getAttribute('start')` returns `''` when the attribute is missing
+and `intval('')` is also `0`.
+
+## The fix
+
+Detect an explicitly provided numeric `start` attribute with `\is_numeric()`
+instead of relying on the truthiness of `intval()`:
+
+```php
+if ($listType === 'ol' && ($parent = $element->getParent()) && \is_numeric($start = $parent->getAttribute('start'))) {
+    $number = (int) $start + $element->getSiblingPosition() - 1;
+} else {
+    $number = $element->getSiblingPosition();
+}
+```
+
+`\is_numeric('')` is `false` (missing attribute -> normal numbering) while
+`\is_numeric('0')` is `true`, so `start="0"` is now respected. Existing values
+such as `start="120"` are unaffected.
+
+## Test
+
+Added an assertion in `HtmlConverterTest::testLists`:
+
+```php
+$this->assertHtmlGivesMarkdown('<ol start="0"><li>Item A</li><li>Item B</li></ol>', "0. Item A\n1. Item B");
+```
+
+## Gate
+
+```
+composer install && vendor/bin/phpunit --no-coverage
+```
+
+Result: exit code `0` - `OK (50 tests, 230 assertions)`.
+
+Counterfactual: reverting `ListItemConverter` to the `intval()` truthiness check
+fails the new assertion - `<ol start="0">` yields `1. Item A\n2. Item B` instead
+of `0. Item A\n1. Item B`.

src/Converter/ListItemConverter.php

@@ -52,8 +52,8 @@ class ListItemConverter implements ConverterInterface, ConfigurationAwareInterfa
             return $prefix . $this->listItemStyle . ' ' . $value . "\n";
         }
 
-        if ($listType === 'ol' && ($parent = $element->getParent()) && ($start = \intval($parent->getAttribute('start')))) {
-            $number = $start + $element->getSiblingPosition() - 1;
+        if ($listType === 'ol' && ($parent = $element->getParent()) && \is_numeric($start = $parent->getAttribute('start'))) {
+            $number = (int) $start + $element->getSiblingPosition() - 1;
         } else {
             $number = $element->getSiblingPosition();
         }

tests/HtmlConverterTest.php

@@ -192,6 +192,7 @@ class HtmlConverterTest extends TestCase
         $this->assertHtmlGivesMarkdown('<ol><li>   Item A</li><li>   Item B</li></ol>', "1. Item A\n2. Item B");
         $this->assertHtmlGivesMarkdown('<ol><li>  <h3> Item A</h3><p>Description</p></li><li>   Item B</li></ol>', "1. ###  Item A\n    \n    Description\n2. Item B");
         $this->assertHtmlGivesMarkdown('<ol start="120"><li>Item A</li><li>Item B</li></ol>', "120. Item A\n121. Item B");
+        $this->assertHtmlGivesMarkdown('<ol start="0"><li>Item A</li><li>Item B</li></ol>', "0. Item A\n1. Item B");
         $this->assertHtmlGivesMarkdown('<ul><li>first item of first list</li><li>second item of first list</li></ul><ul><li>first item of second list</li></ul>', "- first item of first list\n- second item of first list\n\n* first item of second list", ['list_item_style_alternate' => '*']);
     }