I had to hide some iframes recently and couldn't quite work out the styling I needed to apply

The normal display: none and other custom CSS often doesn't work (because of the cascasing default styled you'll get borderds still applied etc)

So the style you need is govuk-visually-hidden, and make sure you hide it via aria too

<p class="govuk-visually-hidden" aria-hidden="true">This is my hello world p tag</p>