WordPress HTML Minification Class to compress HTML (removal of whitespaces, line breaks, new lines, comments). Preserves script comments (if removed might break some javascripts like Google Analytics or Adsense) and IE specific tags.

functions.php

/* 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 
 * 
 * @see http://forrst.com/posts/Wordpress_Minify_output_HTML-29q
 * @see http://www.intert3chmedia.net/2011/12/minify-html-javascript-css-without.html 
 */
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' );

Replace get_header of after_setup_theme.

It was removing the schema added to html is there any possibility to do without affecting schema ?