Note: this will be deprecated upon the release of WordPress 5.0 as the plugin will be integrated into WordPress core.
WordPress sites using threaded comments are advised to include the comment-reply.js script included in WordPress core. As the script is included in every version of WordPress, the theme developer can simply enqueue the script if required and WordPress does the rest.
Unfortunately, the version of the script included in WordPress core is not unobtrusive. The typical comment reply link in WordPress looks something like this:
<a class='comment-reply-link' href='/sample-post/?replytocom=1#respond' onclick='return addComment.moveForm( "div-comment-1", "1", "respond", "6" )' aria-label='Reply to John Smith'>Reply</a>
<a class='comment-reply-link' href='/sample-post/?replytocom=1#respond' data-comment-id="1" data-post-id="6" data-add-below-element="div-comment-1" data-respond-element="respond" aria-label='Reply to John Smith'>Reply</a>