index.php

<?php

// that class is usually used, if you work with html in your view.
use yii\helpers\Html;

// here comes your Yii2 asset's class!
use app\modules\YOUR-MODULE\assets\YOUR-ASSER-CLASS;

// now Yii puts your css and javascript files into your view's html.
MyModuleAsset::register($this);
?>

<p>
  Some <strong>HTML</strong> my <i>friend..</i> :)
</p>