It’s more important than ever ensure all aspects of your site are optimised including your raw HTML and inline Javascript and CSS. I will share a function I use on on my sites in a MU-Plugin, using WordPress functions we can remove all whitespace and comments from our HTML output. This function also preserves essential comments for IE, Google Analytics and comments within script tags.

You can add this function to a mu-plugin to use on your site or multisite network. You can even disable compression on a select portion of code by using this tag before the code

<!--wp-html-compression no compression-->

Here is the function to minify your HTML, Javascript and CSS on WordPress:

<?php
/**
 * Plugin Name: Lakewood Minify
 *
 * Minifies HTML and removes comments (except IE tags and comments within script tags)
 * 
 * To disable compression of code portions, use '<!--wp-html-compression no compression-->' tag 
 *
 */
class WP_HTML_Compression
{
        // Settings
        protected $compress_css = true;
        protected $compress_js = false;
        protected $info_comment = true;
        protected $remove_comments = true;

        // Variables
        protected $html;
        
        public function __construct($html)
        {
                if (!empty($html))
                {
                        $this->parseHTML($html);
                }
        }
        
        public function __toString()
        {
                return $this->html;
        }
        
        protected function minifyHTML($html)
        {
                $pattern = '/<(?<script>script).*?<\/script\s*>|<(?<style>style).*?<\/style\s*>|<!(?<comment>--).*?-->|<(?<tag>[\/\w.:-]*)(?:".*?"|\'.*?\'|[^\'">]+)*>|(?<text>((<[^!\/\w.:-])?[^<]*)+)|/si';                
                preg_match_all($pattern, $html, $matches, PREG_SET_ORDER);
                $overriding = false;
                $raw_tag = false;
                // Variable reused for output
                $html = '';
                foreach ( $matches as $token ) {
                    
                        $tag = (isset($token['tag'])) ? strtolower($token['tag']) : null;    
                        $content = $token[0];
                        
                        if ( is_null( $tag ) ) {
                            
                                if ( !empty( $token['script'] ) ) {
                                    
                                        $strip = $this->compress_js;
                                        
                                } else if ( !empty($token['style'] ) ) {
                                    
                                        $strip = $this->compress_css;
                                        
                                } else if ( $content == '<!--wp-html-compression no compression-->' ) {
                                    
                                        $overriding = !$overriding; 
                                        // Don't print the comment
                                        continue;
                                        
                                } else if ( $this->remove_comments ) {
                                    
                                        if ( !$overriding && $raw_tag != 'textarea' ) {
                                            
                                                // Remove any HTML comments, except MSIE conditional comments
                                                $content = preg_replace('/<!--(?!\s*(?:\[if [^\]]+]|<!|>))(?:(?!-->).)*-->/s', '', $content);                                                
                                        }
                                }
                                
                        } else {
                            
                                if ( $tag == 'pre' || $tag == 'textarea' || $tag == 'script' ) {
                                    
                                        $raw_tag = $tag;
                                        
                                } else if ( $tag == '/pre' || $tag == '/textarea' || $tag == '/script' ) {
                                    
                                        $raw_tag = false;
                                        
                                } else {
                                        
                                        if ($raw_tag || $overriding) {
                                            
                                                $strip = false;
                                                
                                        } else {
                                            
                                                $strip = true;
                                                
                                                // Remove any empty attributes, except:
                                                // action, alt, content, src
                                                $content = preg_replace('/(\s+)(\w++(?<!\baction|\balt|\bcontent|\bsrc)="")/', '$1', $content);
                                                
                                                // Remove any space before the end of self-closing XHTML tags
                                                // JavaScript excluded
                                                $content = str_replace(' />', '/>', $content);                                                
                                        }
                                        
                                }
                                
                        }
                        
                        if ( $strip ) {
                            
                                $content = $this->removeWhiteSpace($content);                                
                        }
                        
                        $html .= $content;                        
                }
                
                return $html;
        }
                
        public function parseHTML($html)
        {
                $this->html = $this->minifyHTML($html);
        }
        
        protected function removeWhiteSpace($str)
        {
                $str = str_replace( "\t", ' ', $str );
                $str = str_replace( "\n",  '', $str );
                $str = str_replace( "\r",  '', $str );
                
                while ( stristr($str, '  ' ) ) {
                    
                        $str = str_replace('  ', ' ', $str);
                }
                
                return $str;
        }
}

function wp_html_compression_finish($html) {
    
        return new WP_HTML_Compression($html);
}

function wp_html_compression_start() {
    
        ob_start( 'wp_html_compression_finish' );        
}

add_action( 'get_header', 'wp_html_compression_start' );