Automatically bold first paragraph of content?

You can use CSS3 :first-child pseudo-class to achieve this.
https://www.w3schools.com/css/pr_pseudo_first-child.asp

It won’t work in IE and old browsers though.

1. Automatically add a class (e.g.: class=”lead”) to your first paragraph, you can do it this way:

function first_paragraph($content){
	return preg_replace('/<p([^>]+)?>/', '<p$1 class="lead">', $content, 1);
}
add_filter('the_content', 'first_paragraph');

source: https://36flavours.com/2009/08/wp-add-class-name-to-first-paragraph-in-blog-post/comment-page-1/#comment-994

2. In your style sheet add:

p.lead {
  font-weight: bold;
}

Not working at all for me.

I know that the code is being parsed because when I took the <?php bit off of the front end of the file the contents of the file showed up at the top of my site.

My mistake! It was a conflict with a plugin that was preventing the function from formatting the post.