Add block in <head> tag in Magento 2
Replace this following code in your Layout Xml. This will work for you.
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="head.additional">
<block class="Magento\Framework\View\Element\Template" name="frontuser" template="Frontuser_Tags::frontuser/tags.phtml"/>
</referenceBlock>
</body>
</page>
Hope this helps you
Use
<referenceBlock name="head.additional">
Instead of
<referenceContainer name="after.body.start">