ckeditor/ckeditor5-engine

<style>
    .ck-content h2.my-heading {
        font-family: Georgia, Times, Times New Roman, serif;
        border-left: 6px solid #fd0000;
        padding-left: .8em;
        padding: .1em .8em;
    }
</style>

<div id="snippet-heading-class">
    <h2>Heading with <code>.my-heading</code> class</h2>
    <h3>Regular heading</h3>
    <p>Some content.</p>
</div>