The onRenderHtml() hook defines how pages are rendered to HTML.
The onRenderHtml() and onRenderClient() hooks are essentially the glue code between Vike and the UI framework (React/Vue/Solid/...).
If you use a UI framework Vike extensionvike-react/vike-vue/vike-solid, then you don't need to (and shouldn't) define onRenderHtml() as it's already defined by vike-react/vike-vue/vike-solid.
Where pageContext.Page is the +Page value of the page that is being rendered.