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I did a search but nothing showed that resembled this idea, I must admit I have not extensively looked through the HTML5 specs so maybe luckily this is already in the specs. A major issue currently in browsers encountering HTML5 is how to treat unsupported tags. Should they ignore the opening and closing tags and render the content. (current HTML behavior) But this breaks styling, so some many browsers (but not IE?) allow styling of the content of unsupported tags. My idea is that all browsers supporting HTML5 or partially HTML5 "must" treat all unknown tags as if they where <div> tags. This should ensure DOM trees, CSS styling, and semantics are minimally affected. This should also (where possible) be retroactively patched into non HTML5 browsers. Alternatively HTML5 browsers could also simply keep a list of <div> synonyms, so that if <section> is not supported it would be treated as a synonym for <div>, later when <section> is supported it would be removed from the synonym list obviously. Such tag synonym lists would be very easy for browser developers to add and maintain as new HTML5 (and HTML6) tags are defined but not yet supported/implemented. Most browsers today do have such a behavior for certain tags that has identical behavior. So to reiterate: The specs should state that any unsupported tags must be treated as if they where <div> tags, unless the unsupported tag is treated as a synonym by the browser for another tag while awaiting final definition of the new tag's behavior in the specs. Does this make sense, or...?
HTML5 actually does define how you handle unknown elements; they're more or less treated exactly like <span>. It's not really explicit anywhere, more an implicit result of defining a lot of things via defaults (e.g. parsing behaviour for <bdo> isn't mentioned explicitly, so it uses the defaults, which is the same as for unknown elements).
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