Google crawled in 2005 around a billion pages to find out what Tags and Tag attributes are used within them.
General
So this Study shows for example that within the <html> the attribute xml:lang is quite often used, but it’s totally senseless within a HTML Document as no HTML processor does look at it.
It also shows that many people aren’t able to write values within quotes, so mostly wrong attributes and values for them would be the answer.
As you also can find out is that the meta name revisit-after is useless as just one Spider ever looked at this attribute.
Do we need this for SEO?!
What I personally found really interesting is this Sentence about meta attributes:
Next we have two name values: keywords, which these days is mostly useless, ironically, and description, which is still somewhat useful.
Does that mean that parts of SEO, like the meta SEO is totally senseless, as these tags aren’t taken serious any more?
The Look is the Content
As we already know Tables are misused by many websites and templates for designing purpose, therefore most attributes uses within them are just for presentation. But that applies also for to the body tag. As you also can see from these results most pages still use the attributes to define the Styling rather than css to do this.
Hyper Text Mark-up Langauge is Difficult
It’s really interesting to see how many misspell attributes just one example is the above already misspelled Language which can be found in a lot of versions on the internet. And what I didn’t know for myself you don’t need to add even the language attribute to the script tag, as there’s just one valid language for it!
I need my own Tag attribute
The last thing I want to mention is the quite big amount of self invented tags, which comes from Adobe GoLive, MicroSoft Office and some more. Where I can’t see any sense behind it as these tags aren’t supported by Browser, or mostly not supported. So what are they for?
Conclusion
To write 100% HTML 4.01 Mark-up is a lot of work and needs much attention. Not only when you have written it also after you published it. Maybe HTML isn’t for everyone, and you may need a HTML Expert, to be able to publish HTML code.
Take a look at the Study for yourself, it’s worth looking.