As explained by WebAIM, using proper heading tags in the correct order helps provide context and makes websites more accessible. To encourage this accessible practice, it should be mentioned in the <h1> - <h6> section.
Also the example for the <header> tag should be change from
<article>
<header>
<h1>Most important heading here</h1>
<h3>Less important heading here</h3>
<p>Some additional information here</p>
</header>
<p>Lorem Ipsum dolor set amet....</p>
</article>
to
<article>
<header>
<h1>Most important heading here</h1>
<h2>Less important heading here</h2>
<p>Some additional information here</p>
</header>
<p>Lorem Ipsum dolor set amet....</p>
</article>
