CSS препроцессоры, фреймворки

use TijsVerkoyen\CssToInlineStyles\CssToInlineStyles;

// create instance
$cssToInlineStyles = new CssToInlineStyles();

$html = file_get_contents(__DIR__ . '/examples/sumo/index.htm');
$css = file_get_contents(__DIR__ . '/examples/sumo/style.css');

// output
echo $cssToInlineStyles->convert(
    $html,
    $css
);

/*! minireset.css v0.0.2 | MIT License | github.com/jgthms/minireset.css */html,body,p,ol,ul,li,dl,dt,dd,blockquote,figure,fieldset,legend,textarea,pre,iframe,hr,h1,h2,h3,h4,h5,h6{margin:0;padding:0}h1,h2,h3,h4,h5,h6{font-size:100%;font-weight:normal}ul{list-style:none}button,input,select,textarea{margin:0}html{box-sizing:border-box}*{box-sizing:inherit}*:before,*:after{box-sizing:inherit}img,embed,object,audio,video{height:auto;max-width:100%}iframe{border:0}table{border-collapse:collapse;border-spacing:0}td,th{padding:0;text-align:left}