Minify the HTML that is output from WordPress.

functions.php

/**
 * Replace the buffer.
 *
 * @link http://insnvlovn.blogspot.jp/2010/04/php-obstart.html
 * @param  buffer $buffer
 * @return buffer $buffer
 */
function buffer_replace( $buffer ) {
    $pattern = array( "/(?:\r\n)|[\r\n]/", "/[\\x00-\\x09\\x0b-\\x1f]/" );
    $replace = array( "\n", " " );
    $buffer  = preg_replace( $pattern, $replace, $buffer );
    unset( $pattern, $replace );

    $pattern = array( '/<!--(.*?)-->/', '/\n/', '/\t/', '/[　\s]+/umi' );
    $replace = array( '', '', '', ' ' );
    $i       = 0;
    $tmpName = "__TMP__";

    do {
        if ( !isset( $GLOBALS[$tmpName . $i] ) ) {
            $tmpName .= $i;
            break;
        }
        if ($i > 10) {
            $tmpName .= md5( mt_rand() ) . md5( mt_rand() );
            break;
        }
        $i++;
    } while ( true );

    $GLOBALS[$tmpName] = array();
    $buffer            = preg_replace_callback( "/<(pre|s(?:cript|tyle)|xmp)(?:\s*|(?:\s+[^>]+))>(.*?)<\/\\1\s*>/is", create_function( '$matches', '$tmp =& $GLOBALS["' . $tmpName . '"];' . 'if ($matches[2] === "") { return $matches[0]; }' . '$tmp[] = $matches[0];' . 'return "<\\x00," . count( $tmp ) . ",\\x01>";'), $buffer );
    $buffer            = preg_replace( $pattern, $replace, $buffer );
    $buffer            = preg_replace_callback( "/<\\x00,(\d+),\\x01>/", create_function( '$matches', '$tmp =& $GLOBALS["' . $tmpName . '"];' . 'return $tmp[$matches[1] - 1];' ), $buffer );

    return $buffer;
}

/**
 * Turn on output buffering.
 *
 * @return boolean
 */
function buffer_ob_start() {
    ob_start( 'buffer_replace' );
}
add_filter( 'after_setup_theme', 'buffer_ob_start' );

/**
 * Flush (send) the output buffer and turn off output buffering.
 *
 * @return boolean
 */
function buffer_ob_end_flush() {
    ob_end_flush();
}
add_filter( 'shutdown', 'buffer_ob_end_flush', 0 );