About Obsolete Features in HTML5

As many of you may be aware, validating your pages in HTML5 has become a whole new adventure.
Along with changes in what now constitutes “valid” and “invalid” markup, the HTML5 Validator (which, by the way, is the same tool that runs the W3C’s HTML5 validator) will provide some warnings about obsolete features in HTML5.
Feel free to go through that document on the W3C’s site, but here I’ll touch on some things that you may want to be aware of with regards to obsolete elements and attributes.

There’s No More “Deprecated”

Many developers will be familiar with the word “deprecated” when referring to elements and attributes that no longer validate or that are no longer part of the current HTML spec. This was the term used in previous versions of HTML, but not with HTML5.
So, for example, if you validate a document using an XHTML Strict doctype, and you use the

</code> element or the <code>target</code> attribute, the validator will output the two errors shown in the screen grab below:</p>
<hr class="more">
<p>
</p>
<p>
<img alt="Two validation errors with deprecated elements in XHTML" class="wide_image" height="455" loading="lazy" src="https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_smlahKEqZCneFN_VlJFz3xps4_-9pjcZGOK3Qi74es0imh7q_7LsvlzJcaYCgWpMvMjaU3xoDpGu_qdB8lIzdXiqthoUiCRGKlN9UI=s0-d" title="Two validation errors with deprecated elements in XHTML" width="550"></p>
<p>
This has changed to a large degree in HTML5. In the case of <code><iframe></code> and <code>target</code>, those features are now valid in HTML5, so there&#8217;s no problem&#8211;the page will validate and there won&#8217;t be any warnings.</p>
<h2>
Obsolete But Conforming Features</h2>
<p>
Let&#8217;s try instead to validate an HTML5 document that contains 
obsolete features that are still considered valid in HTML5. Here&#8217;s a 
screen grab of the message you&#8217;ll see if you validate a document that 
uses an <code><img loading="lazy"></code> element with the <code>border</code> attribute, in addition to an <code><a></code> element with the <code>name</code> attribute (both attributes of which are now obsolete in HTML5 in those contexts):</p>
<p>
<img alt="Warnings for Obsolete Elements in HTML5" class="wide_image" height="270" loading="lazy" src="https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_teH96o_9KvFLVQZGiACJMZnu0ec4rf4ttqvjtGdttJtmIYKutllOLENEs3JzVcr0-PZKdd15XLXAzIu2ZKFSuRp0e_fuJFUBWi9438rlhsFvn4mg=s0-d" title="Warnings for Obsolete Elements in HTML5" width="550"></p>
<p>
So, according to the HTML5 validator, the page is still valid, but 
you are warned to instead use alternative methods to accomplish the same
 thing.</p>
<p>
For a full list of &#8220;obsolete but conforming&#8221; features see <a href="http://dev.w3.org/html5/spec/obsolete.html#obsolete-but-conforming-features">the appropriate section</a> in the same W3C document.</p>
<h3>
Totally Obsolete Elements</h3>
<p>
On the other hand, some elements are just downright wrong and should 
not be used. So you won&#8217;t just get a warning, you&#8217;ll get a full-blown 
error with a recommendation to accomplish the same thing in a valid 
manner.</p>
<p>
Some of the error-inducing obsolete elements include <code><applet></code>, <code><acronym></code>, <code><frame></code>, <code><noframes></code>, <code><strike></code>, <code><big></code>, and <code><blink></code>. You can see a full list of non-conforming obsolete features <a href="http://dev.w3.org/html5/spec/obsolete.html#non-conforming-features">here</a>.</p>
<h2>
Will Obsolete Features Break Your Pages?</h2>
<p>
Generally speaking, no, these invalid elements will not break your 
pages. The HTML5 doctype is not a doctype to identify a markup language,
 it&#8217;s a doctype to identify a web document, whether it&#8217;s using old 
elements or new stuff like <code><nav></code> and <code><section></code>.</p>
<p>
I know I&#8217;ve probably oversimplified the definition of a doctype here,
 but generally speaking, that&#8217;s what is happening. That&#8217;s why it&#8217;s okay 
to change old documents to an HTML5 doctype without fear of your pages 
breaking or rendering differently.</p>
<p>
Also, browser makers <i>are required</i> to support those old obsolete elements. That same document contains a section called <a href="http://dev.w3.org/html5/spec/obsolete.html#requirements-for-implementations">Requirements for Implementations</a>, which basically tells browser makers how obsolete elements should be handled when the page is rendered.</p>
<p>
Of course, when we say that browser makers are &#8220;required&#8221; to do this,
 it&#8217;s not as if they&#8217;ll be arrested or fined if they don&#8217;t; it&#8217;s really 
just a recommendation, but I think for the most part browser makers have
 complied.</p>
<p>
I guess if anything, this should help further drive home the point 
that it&#8217;s okay to to start changing pages with old doctypes to the HTML5
 doctype. It&#8217;s true that the pages may not technically validate anymore 
(in most cases they will), but they will still render the same.</p>

About Obsolete Features in HTML5

There’s No More “Deprecated”

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