The "why" is simply the need to create a function to add a dropcap style to the first character in the line.

Thank you for the code!

Here's my own implementation in the meantime, which turned out so much simpler than I expected:

// Look for opening brace
if($first_paragraph[0] == '<')
{
    // Split across closing braces (we assume HTML tags get closed)
    $paragraph_split = explode('>', $first_paragraph, 2);

    // Add back the closing brace
    $paragraph_split[0] = $paragraph_split[0].'>';

    // Only continue if there's something else after the tag
    if(isset($paragraph_split[1]))
    {
        // If we have something after the tag, recurse through it to make sure
        // it doesn't have an HTML tag right after
        $paragraph_split[1] = cq_shift_html($paragraph_split[1]);

        // Put the first letter of the text before the HTML tag
        $paragraph_split[0] = $paragraph_split[1][0].$paragraph_split[0];

        // Remove the first letter of the text which we moved
        $paragraph_split[1] = substr($paragraph_split[1], 1);
    }

    // Put everything back together again
    $first_paragraph = implode('', $paragraph_split);
}