serverMiddleware are just running in server side before vue-server-renderer and can be used for server specific tasks like handling API requests or serving assets.
По CSS вопросов нет - там просто соблюдать "чистоту"
<!--[if lt IE 9]>
<script src="libs/html5shiv/es5-shim.min.js"></script>
<script src="libs/html5shiv/html5shiv.min.js"></script>
<script src="libs/html5shiv/html5shiv-printshiv.min.js"></script>
<script src="libs/respond/respond.min.js"></script>
<![endif]-->