I'm currently developing a website using h3, h4, h5 only (based on labels from the design department). The header hierarchy itself is fitting but I'm wondering if it would have a positive effect on SEO to shift the hierarchy upwards to h1, h2, h3.
Google recommends observing the hierarchy in the structure of heading elements:
Use heading tags to structure your content hierarchically - for example,
Mozilla recommends the same:
A common navigation technique for users of screen reading software is jumping from heading to heading to quickly determine the content of the page. Because of this, it is important to not skip one or more heading levels.
The correct hierarchical structure of your HTML document can help you create a structured data tree for that document.
It is always best practice to construct documents semantically, and in accordance with the latest standards.
If you're publishing high quality content that is valuable to users, it will rank high on search engines.
Let's say you wrote an amazing blog post that was much better than everything out there on the subject.
Google, for example, isn't going to be like "wow this is spectacular it should rank page 1...wait a sec...scratch that they put an
<h3> before an
<h1>...put it on page 3."
Would it be best practice to construct your headings semantically?
Are your pages going to rank any higher than they do now after you implement that change?
I Highly doubt it.
In my testing, I haven't observed a statistically significant positive correlation between the order of
<h3> and average position increase since ~2016.
Google primarily cares about how your pages look to a user. As long as that they can be crawled, they can rank. Quality content is a must have, 100% optimized on-page factors are nice but not required.
That's not to say on-page SEO is dead.
If in addition to non-semantic headings, you only had one image and it didn't have alt text, your internal links were all naked URLs, the text was too small to read, you didn't list a meta description and so forth, then yeah you'd have some issues.
There are some situations where the semantics of your markup could improve how Google dispays your content on search result pages. Providing structured data / generating rich snippets are an example, which I believe Nikant25's answer alludes to.