In an interview at Tech Republic, HTML5 Editor Ian Hickson stated:
The second big controversy in recent history was over extensibility. There have been some requests to allow people to extend HTML without speaking to the committees working on HTML. We’ve provided a number of mechanisms for this (the class and rel attributes, the data-* attributes, the meta element for page-wide metadata, the script element for non-script data blobs, the embed element for plugins), but some people simply want the ability to invent their own elements and tag names. So far, we’ve managed to avoid that, and we’ll have to see if we can continue.
Yes, but we’ve still not resolved—at least, I’m not aware of any resolution—about how to incorporate MathML, RDFa, and SVG into HTML5. I can’t help thinking we’ve spent more time trying to prohibit extensibility than we would if we just provided the mechanism.
In addition, I’m frankly confused about how we’ll pull off a consistent model between HTML5, which is rigidly inflexible, and XHTML5, which is anything but. If what’s incorporated into HTML5 differs from what’s allowed in XHTML5, do we just…wing it?
The whole point of XML years ago was because of issues like this—how do we create a markup that can be extended without having to update the underlying specification/model with each new addition. Ever since, we’ve been running in horror from the Yellow Screen of Death—the negative aspect of XML we fixate on—while arguing, endlessly, about extensibility—the most beneficial aspect of XML. I can’t help thinking that we should keep the extensibility and just get rid of the Yellow Screen of Death.
However, I’m not expert in these things. I am just a humble web developer, with simple needs. One such need is I don’t want to lose what I have now.