There are very few Blogger.com sites that do validate, they are usually very minimalistic layouts. However, Wordpress.com pages seem to validate like cake right out of the box.
I casually checked out my homepage for this blog at validator.w3.org, I wasn't asking for any trouble but all of a sudden things got ugly:
676 Errors, 145 warning(s)
My next couple of points reference Blogger or Wordpress? Which is more SEO compatible from seowizz.net
Tim from seowizz.net points out that Blogger jacks control of your h1 tags on every page, so while everyone else is getting help ranking for their keywords in h1 tags, Blogspot bloggers are forced to start with h2 headings to help rank the keywords in their posts. The h1 heading on Blogger blogs pages all belong to the blogs title and there is no way around it. Tim shows us why this is a problem -
"The H1 tag is the main description of your on page content after your title tag, therefore it is important to make it diverse throughout your pages." From seowizz.net
Also, in the same post Tim points out that Blogger didn't get the memo about 'code to content' ratio and how storing CSS on the page dramatically increases the amount of code the spiders have to sift through to get to the content.
"This becomes an issue on blogger as all your CSS elements are on your pages, this seriously increases your code to content ratio " From seowizz.net
There seems to be alot of SEO issues with Blogger, I truely believe new search concious bloggers looking for a blogging platform should choose Wordpress. Alot of people seem to think Google gives a little ranking boost to Blogger blogs since Blogger is owned by Google - this is dangerous nonsense and no SEO has provided any evidence to validate such a theory.