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Im pretty sure you don't need to do it into 'root' how you have done it.

I would think it is like so.

<referenceContainer name="page.wrapper">
    <container name="some.container" as="someContainer" label="Some Container" htmlTag="div" htmlClass="some-container" after="header.container" />
</referenceContainer>

Add this into:

Vender/theme/Magento_Theme/layout/default.xml

Or for root it might be:

<container name="root">
        <container name="some.container" as="someContainer" label="Some Container" htmlTag="div" htmlClass="some-container" after="header.container" />
    </container>