Translatewiki has $wgExperimentalHtmlIds = true.
https://translatewiki.net/w/api.php contains:
<h3 id="main/datatypes" class="apihelp-header"><span id="main/datatypes"></span> [...] <h3 id="main/credits" class="apihelp-header"><span id="main/credits"></span>
This contains duplicate IDs: main/datatypes and main/credits.
https://validator.w3.org/nu/?doc=https%3A%2F%2Ftranslatewiki.net%2Fw%2Fapi.php generates:
Error: Duplicate ID main/datatypes. From line 59, column 48; to line 59, column 73 p-header"><span id="main/datatypes"></span Warning: The first occurrence of ID main/datatypes was here. From line 59, column 1; to line 59, column 47 l></div>↩↩<h3 id="main/datatypes" class="apihelp-header"><span Error: Duplicate ID main/credits. From line 74, column 96; to line 74, column 119 p-header"><span id="main/credits"></span Warning: The first occurrence of ID main/credits was here. From line 74, column 51; to line 74, column 95 </dd></dl><h3 id="main/credits" class="apihelp-header"><span