This question follows directly from the part of an answer of this question - Why would one bother marking up properly and semantically? .
I would like to understand why SEO techniques gives more weight to semantically correct mark up?
What does the semantic markup offer? A way for the machines to understand the data. What's a search engine? A machine.
Back in the days, the search engines just looked for the words in a page and showed the page when you search the keywords. However, this led to farm websites with thousands of keywords just to rank higher than anything else, but this wasn't a quality website.
So the search engines evolved, adding more and more to their algorithm in order to provide relevant results to the end-user.
And then, search engines added a new way for websites to tell what they are: semantic markup. When a spider sees a specific tag, it knows what it means. The word "meaning" has a lot of importance there.
Lately, SEO is not only about how to rank the highest. Sure, it's quite important, but it can be reached only through great content, internal links, backlinks, and a lot of work overall (semantic markup is also a plus there, but it's not so much according to my experience).
There are other ways to be seen though: using semantic markup. Because Google can now link your result to a picture of your face, because it can also show a map of where your hotel is, because it can see your post is relevant to this date, etc. The point is not to be the highest ranked; it is about the visibility. Semantic markup is what allows your website to be visible on search engines.
Just try it: Google some hotel's name around your place: you'll get the phone number and a Google Maps with its location, although it might not be the first result. That's visiblity.
The text that the question refers to is based on a thorough confusion, rooted in the vagueness (or semantic obscurity!) of the slogan “semantic markup.”
On one hand, “semantic markup” is markup that someone chooses to call “semantic”, and this may mean anything that is not directly defined in terms of rendering. (The HTML5 drafts even try to make
<i> semantic by redefining it.) Typically, this nowadays means favoring novelties like
On the other hand, there is metadata attached to elements using markup like microdata or microformats, i.e. using reserved
class names or some new attributes tailored specifically for the purpose. Such metadata is recognized by some major search engines – it was more or less designed by them – under conditions that they do not disclose (but which appear to be currently restricted to major commercial sites and most popular social media or other community sites).
No factual evidence has been given about search engines actually paying the least attention to “semantic markup” in the first sense, which is far more commonly used.
why SEO techniques gives more weight to semantically correct mark up ?
Your question is actually addressed in a reference to - Semantic HTML:
In 2010, Google specified three forms of structured metadata that their systems will use to find structured semantic content within webpages. Such information, when related to reviews, people profiles, business listings, and events will be used by Google to enhance the 'snippet', or short piece of quoted text that is shown when the page appears in search listings. Google specifies that that data may be given using microdata, microformats or RDFa. Microdata is specified inside
itempropattributes added to existing HTML elements; microformat keywords are added inside
classattributes as discussed above; and RDFa relies on
propertyattributes added to existing elements.
I would like to understand why SEO techniques gives more weight to semantically correct mark up ?
Because (certain) search engines pay attention to semantic markup when fulfilling search requests.
SEO is a set of techniques that try to make your website rank higher than others in people's search results. This is done by exploiting knowledge of how various search engines rank the raw search results. If (say) Google takes semantic markup into account, then an SEO optimizer might add semantic markup to improve the relevance of a site or page.
A more salient question would be to ask why search engines like Google bother to pay any attention to semantic markup if people are exploiting it for SEO purposes. Presumably there is enough useful / good semantic markup around to make it worthwhile ... and to counter the misleading / bad stuff that shonky SEO outfits might create.
For companies like Google information is money and the better their information is the more value it has. Pages with semantically correct mark up contain more and better information then pages without.
So if Google wants to promote page-makers to use semantic markup, the easiest way to do that is to rank pages with semantic markup higher.