To prevent this happening but leave the auto conversion of double line-breaks into paragraph tags, I had a dig into the wpautop function that runs before the content is outputted to the screen.

This function accepts two parameters, the first is the content to be formatted and the second determines whether to convert remaining line breaks into <br /> tags.

Because the wpautop filter is automatically applied to the content, we first need to remove that by adding the following to our functions.php file:

remove_filter('the_content', 'wpautop');

Then we must apply the formatting function again but this time set the the second parameter to false and prevent the additional <br /> tags appearing.

function wpautopnobr($content) {
	return wpautop($content, false);
}
add_filter('the_content', 'wpautopnobr');

This can be customised further by checking to see if we are on a page rather than a post and only applying the formatting if that is the case. The required code in it’s entirety is as follows:

remove_filter('the_content', 'wpautop');
function wpautopnobr($content) {
	return wpautop($content, (is_page() ? true : false));
}
add_filter('the_content', 'wpautopnobr');

Instead I decided to write a very simple function to add a class name to the first paragraph by filtering the content, then setting the styles accordingly in my theme CSS file.

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

By default this will append class=”first” to the first p tag in your post.

If you want to use this function on your own blog, just add it to your (or create a)  functions.php file within your themes folder.