Custom tags... why not?

I found a site with an guide to add custom tags to html, the same way people make ie work with the new HTML5 tags. I must admit I think it would be great to add my own tags, would make it easier to "scan" the the code, and find what you are looking for. But every site I found about it, people say it's not good.... but why isn't it good?

Example html with class:

<ul class="commentlist">
    <li class="comment odd">
    <div class="1">
        <div class="avatar">
            <img src="http://placehold.it/60x60" width="60" height="60" />
        </div>
        <div class="metadata">
            <div class="name">Name</div>
            <p>response1</p>
        </div>
    </div>
    <ul class="children">
        <li class="comment even">
            <div class="2">
                <div class="avatar">
                    <img src="http://placehold.it/60x60" width="60" height="60" />
                </div>
                <div class="metadata">
                    <div class="name">Name</div>
                    <p>response1a</p>
                </div>
            </div>
        </li>
        <li class="comment odd">
            <div class="3">
                <div class="avatar">
                    <img src="http://placehold.it/60x60" width="60" height="60" />
                </div>
                <div class="metadata">
                    <div class="name">Name</div>
                    <p>response1b</p>
                </div>
            </div>
        </li>
    </ul>
</li>

And here what I could do with custom tags, I think that would be much easier to find my way around, so why not:

<clist>
<ccommentbox class="odd">
    <ccomment class="1">
        <cavatar>
            <img src="http://placehold.it/60x60" width="60" height="60" />
        </cavatar>
        <cdata>
            <cname>Name</cname>
            <ctext>response1</ctext>
        </cdata>
    </ccomment>
    <cchildren>
        <ccommentbox class="even">
            <ccomment class="2">
                <cavatar>
                    <img src="http://placehold.it/60x60" width="60" height="60" />
                </cavatar>
                <cdata>
                    <cname>Name</cname>
                    <ctext>response1a</ctext>
                </cdata>
            </ccomment>
        </ccommentbox>
        <ccommentbox class="odd">
            <ccomment class="3">
                <cavatar>
                    <img src="http://placehold.it/60x60" width="60" height="60" />
                </cavatar>
                <cdata>
                    <cname>Name</cname>
                    <ctext>response1b</ctext>
                </cdata>
            </ccomment>
        </ccommentbox>
    </cchildren>
</ccommentbox>

Solution

Custom tags are not evil

just consider this:

They are not recognized in IE 6-8 by default -> you have to use JavaScript to introduce each custom tag you use on the page e.g: document.createElement('custom-tag') This means your website will only render correctly with JavaScript turned on
In most browsers your custom tags will be treated as inline elements like <span>, this means you have to write CSS to declare them as custom-tag { display: block }
There is no resource I found that could proof that custom tags have any negative impact on search engines. In fact Google released Angular.js which promotes custom tags (<pane> and <tab>) in its examples on the front page.
Most HTML Editors will mark your custom tags as invalid HTML, because they are not in the official spec.

To summarize:

Use custom tags when there are important elements that have more meaning than <div> and there is no existing HTML 4/5 equivalent. This is especially true for web applications which parse the DOM for special tags/attributes/classes to create components (like Angular.js).
If all you build is a simple website with normal content, stick to standard HTML. You will want your website to work also without JavaScript turned on.
If you build a web application where custom tags could really help to make the source cleaner and express special semantics, use them. All negative implications mentioned above (JavaScript has to be turned on / CSS declaration) won't matter for these cases. The same rules apply to custom attributes.

For more details on this topic: IE compatibility for Angular.js