Since last Google's updates and slashing down many off page shady practices, I'd say on-page now is far more important for your SEO than before. You can read about more detailed here:
Also here's great tips I found very useful.The good thing about on-page today, is that awful lot of tools and plugins these days can help you doing SEO yourself rather than hiring agencies (that if you're willing to delve into the matter yourself.
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