It used to be that all you had to do was write a decent page of keyword-based content, add some meta tags, and then start building links. If you were even halfway good at it, you could expect to achieve respectable rankings. SEO is a lot harder now.
Specifically, on-page SEO is a lot harder now. And it’s getting harder.
What’s making on-page SEO so hard? Why is it getting harder?
There are several reasons why on-page SEO is getting more difficult with each passing day. For starters, Google changes its search algorithms more than 50 times a day, so it’s near impossible to keep up with the changes.
Secondly, there are so many search factors to keep up with that no one can feasibly master them all. And we can’t be sure any more just how much weight is given to specific on-page factors such as keyword density, keywords in subheads, meta tags, page titles, etc. Plus, the addition of schemas and structured data means that some SEO factors may be subject to certain conditions and your rankings may or may not have to do with anything related to those conditions.
For instance, all else being equal, if you use a particular bit of structured data and your competitor doesn’t use any, your competitor could still rank higher for you on some search queries even if you rank higher than him on others.
SEO is getting to be more and more subjective all the time – subjective in the sense that each page is judged on its own merits without consideration for what’s going on in other parts of the web.
There are basics to on-page SEO that every webmaster should pay attention to, but beyond those, your best bet is to test, experiment, and measure. No two web pages are a like and no two search queries are either.