why these sentence appears in bold font and big size in the browser?

Question

<article >
<h1> hello </h1>
<section>
<h1> Heading </h1>
<p> content and image </p>
</section>
<section>
<h1> History <h1>
<p> my history </p> (this sentence)
</section>
</article>

Answer 1

Because you forgot a / to the h1 tag above this sentence. (You have two opening tag)