A PowerShell script that validates one or more URLs against the W3C HTML validation service. Returns info, warning and error messages. You can also switch on messages for valid pages.

Test-HtmlValidity.ps1

<#
.SYNOPSIS
    Validates one or more URLs against the W3C HTML validation service
.DESCRIPTION
    Validates one or more URLs against the W3C HTML validation service. Returns info, warning and error messages. You can also switch on messages for valid pages. 
.NOTES
    Author: Michaël Hompus
    License: This work is licensed under a Creative Commons Attribution-ShareAlike 2.5 Generic License.
.PARAMETER Urls
    One or more URLs to check for HTML validity.
.PARAMETER OutputValid
    Indicates valid urls will be added to the output.
.PARAMETER Delay
    Changes the deault delay between multiple request to the validation service. Default is 1000ms.
.LINK
    Invoke-RestMethod
    https://gist.github.com/eNeRGy164/ffb410e6dc6acb8610af3e4a1a85437c
.EXAMPLE
    Test-HtmlValidity.ps1 -Urls https://blog.hompus.nl/
.EXAMPLE
    Test-HtmlValidity.ps1 -Urls https://blog.hompus.nl/,https://blog.hompus.nl/about/
.EXAMPLE
    Test-HtmlValidity.ps1 -Urls https://blog.hompus.nl/ -OutputValid
.EXAMPLE
    Test-HtmlValidity.ps1 -Urls https://blog.hompus.nl/ -Delay 2500
#>
[CmdletBinding()]
Param(
    [Parameter(Mandatory = $true, ValueFromPipeline = $true, ValueFromPipelineByPropertyName = $true)]
    [string[]]
    $Urls,
    
    [switch]
    $OutputValid,
    
    [int]
    $Delay = 1000
)

BEGIN {
    if ($null -eq $urls) { 
        $urls = @($input) 
    }

    $first = $true
}

PROCESS {
    Function New-OutputMessage {
        [CmdletBinding()]
        Param(
            [Parameter(Mandatory = $true, Position = 0)]
            [string]$URL,
            [Parameter(Mandatory = $true, Position = 1)]
            [string]$Message,
            [Parameter(Position = 2)]
            [string]$Extract,
            [Parameter(Position = 3)]
            [int]$LastLine,
            [Parameter(Position = 4)]
            [int]$FirstColumn,
            [Parameter(Position = 5)]
            [int]$LastColumn,
            [Parameter(Position = 6)]
            [int]$HiliteStart,
            [Parameter(Position = 7)]
            [int]$HiliteLength
        )

        $msg = @{}
        $msg.URL = $URL
        $msg.Message = $Message
        $msg.Extract = $Extract
        $msg.LastLine = $LastLine
        $msg.FirstColumn = $FirstColumn
        $msg.LastColumn = $LastColumn
        $msg.HiliteStart = $HiliteStart
        $msg.HiliteLength = $HiliteLength

        $object = New-Object –TypeName PSObject –Prop $msg 
        
        Write-Output $object
    }

    Function Write-OutputMessage {
        [CmdletBinding()]
        Param(
            [Parameter(Mandatory = $true, Position = 0)]
            [ValidateSet("Valid","Error","Warning","Info")]
            [string]$Type,
            [Parameter(Mandatory = $true, Position = 1)]
            [PSObject]$Message
        ) 

        # Set default layout
        $defaultProperties = @("URL","Type","Message","Extract")
        $defaultDisplayPropertySet = New-Object System.Management.Automation.PSPropertySet("DefaultDisplayPropertySet", [string[]]$defaultProperties)
        $PSStandardMembers = [System.Management.Automation.PSMemberInfo[]]@($defaultDisplayPropertySet)

        $Message | Add-Member -MemberType NoteProperty -Name Type -Value $Type
        $Message | Add-Member MemberSet PSStandardMembers $PSStandardMembers

        Write-Output $Message
    }
    

    $Urls | ForEach-Object {
        $url = $_

        if ($first) {
            $first = $false
        } else {
            Write-Verbose "Sleeping for $($Delay)ms"
            Start-Sleep -Milliseconds $Delay
        }

        Write-Verbose "Validating $url"
        
        $validation = Invoke-RestMethod -Uri "https://validator.nu/?doc=$([System.Net.WebUtility]::UrlEncode($url))&out=json" -UseBasicParsing
        if ($validation.messages) {
            $validation.messages | ForEach-Object { 
                $message = New-OutputMessage $url $_.message $_.extract $_.lastLine $_.firstColumn $_.lastColumn $_.hiliteStart $_.hiliteLength

                if ($_.type -eq "error") {
                    Write-OutputMessage Error $message
                } elseif ($_.subtype -eq "warning") {
                    Write-OutputMessage Warning $message
                } else {
                    Write-OutputMessage Info $message
                }
            }
        } else {
            Write-Verbose  "Valid, No messages returned"
            if ($OutputValid) {
                $message = New-OutputMessage $url "Valid"
                Write-OutputMessage Valid $message
            }
        }
    }
}