Accessing HTML element properties in javascript

rendered isn't a valid attribute for a HTML element, check the Element - Web APIs | MDN[^] documentation page.

If this is a custom attribute then you should probably prefix it with data- to indicate that it shouldn't be considered an actual DOM attribute, but a transient one. I also found this StackOverflow link[^] which indicates some attributes may be server-side and stripped before they reach the browser, in which case you wouldn't be able to access the attribute at all.

If you want to use the data- approach then you can add an attribute called data-rendered="value" and then access it using the getAttribute() method:

const element = document.getElementById('..');

// check if the attribute exists
if (element.hasAttribute('data-rendered'))
{
  // get the attribute
  const rendered = element.getAttribute('data-rendered');
  ..
  // set the attribute
  element.setAttribute('data-rendered', 'value');
}