The MDN documentation page says:
Permitted content: Fallback content, i.e. content that is normally not rendered, but that browsers not supporting the  element will render. (RSS readers and email clients often show the fallback content instead.)</p> </blockquote> <p>However the W3 and whatwg standards say:</p> <blockquote> <p>An iframe element never has fallback content, as it will always create a nested browsing context, regardless of whether the specified initial contents are successfully used.</p> </blockquote> <p>As a result, the <a rel="nofollow noopener" target="_blank" href="https://validator.w3.org">W3 validator</a> sees fallback content as an error: "Text not allowed in element iframe in this context."</p> <p>I didn't dare to go and edit the page myself because I'm not sure if it is intentional that fallback content is documented, even if it goes against standards.</p> </div> <div> <span class="badge badge-primary">HTML</span> <span class="badge badge-primary">Small</span> <span class="badge badge-primary">P1</span> <div class="float-right"> <a class="text-muted" href="https://www.github.com/mdn/sprints/issues/1883" rel="nofollow noreferrer" target="_blank" onclick="dataLayer.push({'event': 'GAEvent', 'eventCategory': 'click', 'eventAction': 'source_bottom', 'eventLabel': 'Source Bottom', 'eventValue': undefined});">Source</a> </div> </div> </div> <div class="card-footer"> <div class="row"> <div class="col"> <img src="https://avatars1.githubusercontent.com/u/315224?v=4&s=40" style="width:20px; height:20px;" class="mr-2 rounded float-left" alt=" picture"> <strong>marcvangend</strong> </div> <div class="col text-right"> </div> </div> </div> </div> </div> <div class="col-12"> <div id="ph-above-best-comment"> </div> </div> <div class="col-12 mb-4 best-comment"> <div> <h2 class="mb-3"><i class="fas fa-check mr-2"></i>Most helpful comment</h2> </div> <div class="card card-custom border-lg-success mb-4"> <div class="card-body pt-3 pb-3 markdown"> <p>The article should state that fallback content is not allowed, perhaps with a note that older browsers may have once supported it. If specific version ranges of a browser can be determined for support of that feature, it could be added to BCD.</p> </div> <div class="card-footer"> <div class="row"> <div class="col"> <img src="https://avatars2.githubusercontent.com/u/1641439?v=4&s=40" style="width:20px; height:20px;" class="mr-2 rounded float-left" alt=" picture"> <strong>a2sheppy</strong> <span class="text-muted ml-1">on 31 Jul 2019</span> </div> <div class="col text-right"> 👍<span class="ml-2 mr-3">2</span> </div> </div> </div> </div> </div> <div class="col-12"> <div class="mb-4"> <ins class="adsbygoogle" style="display:block" data-ad-client="ca-pub-3835332123789605" data-ad-slot="9295564578" data-ad-format="auto" data-full-width-responsive="true"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); </script> </div> </div> <div class="col-12 d-flex align-items-center mb-4"> <h2>All 5 comments</h2> </div> <div class="col-12"> <div class="card card-custom mb-4"> <div class="card-body pt-3 pb-3 markdown"> <p>The fallback content only applies to really old browsers or some email clients that don’t understand the <code><iframe></code> tag, which was added in <strong>HTML 4</strong>.</p> <p>If you were to use the <strong>W3C validator</strong> with <strong>HTML 3.2</strong> or older, it will tell you that the <code><iframe></code> element doesn’t exist.</p> </div> <div class="card-footer"> <div class="row"> <div class="col"> <img src="https://avatars0.githubusercontent.com/u/3889017?v=4&s=40" style="width:20px; height:20px;" class="mr-2 rounded float-left" alt="ExE-Boss picture"> <strong>ExE-Boss</strong> <span class="text-muted ml-1">on 29 Jul 2019</span> </div> <div class="col text-right"> </div> </div> </div> </div> <div class="card card-custom mb-4"> <div class="card-body pt-3 pb-3 markdown"> <p>Fair point. So the question is: what does MDN intend to document? HTML3.2? HTML4? HTML5? All of them? </p> <p>In any case, I seems wrong to state that there is permitted content for the iframe element. If fallback content happens to work in older browsers that's a nice trick for those who need it, but should it be listed as if it were part of the standard? IMHO the paragraph about permitted content should at least mention that this goes against the standard.</p> </div> <div class="card-footer"> <div class="row"> <div class="col"> <img src="https://avatars1.githubusercontent.com/u/315224?v=4&s=40" style="width:20px; height:20px;" class="mr-2 rounded float-left" alt="marcvangend picture"> <strong>marcvangend</strong> <span class="text-muted ml-1">on 30 Jul 2019</span> </div> <div class="col text-right"> 👍<span class="ml-2 mr-3">1</span> </div> </div> </div> </div> <div class="card card-custom mb-4"> <div class="card-body pt-3 pb-3 markdown"> <p>The article should state that fallback content is not allowed, perhaps with a note that older browsers may have once supported it. If specific version ranges of a browser can be determined for support of that feature, it could be added to BCD.</p> </div> <div class="card-footer"> <div class="row"> <div class="col"> <img src="https://avatars2.githubusercontent.com/u/1641439?v=4&s=40" style="width:20px; height:20px;" class="mr-2 rounded float-left" alt="a2sheppy picture"> <strong>a2sheppy</strong> <span class="text-muted ml-1">on 31 Jul 2019</span> </div> <div class="col text-right"> 👍<span class="ml-2 mr-3">2</span> </div> </div> </div> </div> <div class="card card-custom mb-4"> <div class="card-body pt-3 pb-3 markdown"> <p>Well, that list would only include browsers that are older than <strong>HTML 4</strong>, and therefore don’t have an <code><iframe></code> implementation, which results in them treating it as an unknown element.</p> </div> <div class="card-footer"> <div class="row"> <div class="col"> <img src="https://avatars0.githubusercontent.com/u/3889017?v=4&s=40" style="width:20px; height:20px;" class="mr-2 rounded float-left" alt="ExE-Boss picture"> <strong>ExE-Boss</strong> <span class="text-muted ml-1">on 31 Jul 2019</span> </div> <div class="col text-right"> </div> </div> </div> </div> <div class="card card-custom mb-4"> <div class="card-body pt-3 pb-3 markdown"> <p>I've just updated the permitted content to "None" — I think it is confusing to state what the behavior was in an old version of HTML that browsers don't support any more.</p> </div> <div class="card-footer"> <div class="row"> <div class="col"> <img src="https://avatars3.githubusercontent.com/u/47647?v=4&s=40" style="width:20px; height:20px;" class="mr-2 rounded float-left" alt="chrisdavidmills picture"> <strong>chrisdavidmills</strong> <span class="text-muted ml-1">on 28 Aug 2019</span> </div> <div class="col text-right"> 👍<span class="ml-2 mr-3">1</span> </div> </div> </div> </div> </div> <div class="col-12"> <div class="card card-custom mb-4"> <div class="card-body pt-3 pb-3 markdown text-center helpful"> <div class="title">Was this page helpful?</div> <div class="mt-1" onMouseLeave="rating(474006111, 0);"> <i class="fas fa-star inactive" id="star-1" onMouseOver="rating(474006111, 1);" onclick="rate(474006111, 1);"></i> <i class="fas fa-star inactive" id="star-2" onMouseOver="rating(474006111, 2);" onclick="rate(474006111, 2);"></i> <i class="fas fa-star inactive" id="star-3" onMouseOver="rating(474006111, 3);" onclick="rate(474006111, 3);"></i> <i class="fas fa-star inactive" id="star-4" onMouseOver="rating(474006111, 4);" onclick="rate(474006111, 4);"></i> <i class="fas fa-star inactive" id="star-5" onMouseOver="rating(474006111, 5);" onclick="rate(474006111, 5);"></i> </div> <div class="description text-small"><span id="rating-val">0</span> / 5 - <span id="rating-count">0</span> ratings</div> </div> </div> <div class="mb-4"> <ins class="adsbygoogle" style="display:block" data-ad-client="ca-pub-3835332123789605" data-ad-slot="3452512275" data-ad-format="auto" data-full-width-responsive="true"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); 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