QuerySelector returns null even though page has completely rendered

I am querying a complex page that is using non standard html elements with standard html elements. The page has completely loaded and I can see the HTML within “elements” when inspecting the page.

HTML (simplified)

<dl class="slds-form">
 <slot class="slds-grid slds-size_1-of-1">
  ...
 </slot>
</dl>

Ran below JS in console

document.querySelector('.slds-form');

Result

null

Is it not possible to query non-standard HTML element?