Meant to be used as a Wordpress MU plugin. Uses output buffering to add a new filter that parses the entire final HTML output of each page.

wp-html-buffer.php

<?php
/**
 * Plugin Name: WP HTML Buffer
 * Description: Uses output buffering to add a new filter that parses the entire final HTML output of each page.
 * Plugin URI: https://briannadeleasa.com
 * Version: 1.0.0
 * Author: Brianna Deleasa
 * Author URI: https://briannadeleasa.com
 * Text Domain: wp-html-buffer
 */


/**
 * Output Buffering
 *
 * Buffers the entire WP process, capturing the final output for manipulation.
 */

ob_start();

add_action( 'shutdown', function() {

    $final = '';

    // We'll need to get the number of ob levels we're in, so that we can iterate over each, collecting
    // that buffer's output into the final output.
    $levels = ob_get_level();

    for ($i = 0; $i < $levels; $i++) {
        $final .= ob_get_clean();
    }

    // Apply any filters to the final output
    echo apply_filters( 'wp_final_html', $final );

}, 0 );