To answer your case of having pshtml in functions

PSHTMLInFunctions.ps1

Function Write-Header {
 $Header =

    Head{
        '{0}'
        '{1}'
        
    }#end of header



$Content1 = Title "Salutations!"
$Content2 =script -src "MyScript.js"
#$REturn = 
$Header -f $Content1,$Content2
return $REturn
}

Function Write-Body {
    $MyBody = div {
        h1 "Main Title"
        h2 "SubTitle"
        div -Class "Main" -Content {
            p {
                "plop wop wap"
            }
            P {
                "Paragraph 2"
            }
        }
    }

$Body = Body{
    '{0}'
}#End of body

$Body -f $MyBody
    
}

Function Write-Footer {
   
    $Content = div{
        h1 "This is my title"
        p {
            "Woop"
        }
    }
$HtmlContent = 
    Footer{
        '{0}'
    }#End of footer

return $HtmlContent -f $Content
}

#your Script

Html {

    Write-Header
    Write-Body
    Write-Footer
}