There are several good writeups of the HTML5 WebClient already, like Freek Berson’s: http://microsoftplatform.blogspot.com/2018/01/html5-client-for-microsoft-remote.html
This post will simply show the installation on a Windows Server 2019 Remote Desktop Services deployment.
The Microsoft guide for installing the client can be found here: https://docs.microsoft.com/en-us/windows-server/remote/remote-desktop-services/clients/remote-desktop-web-client-admin
So lets start by following the guide on Windows Server 2019.
First of all, find the certificate that is used by your RD Connection Brokers and export this to a BASE64 encoded .cer file.
Open an elevated PowerShell prompt and update the PowerShellGet module as you would on Windows Server 2016:
Install-Module -Name PowerShellGet -Force
Open a new elevated PowerShell prompt and install the RDWebClientManagement module:
Install-Module -Name RDWebClientManagement
When it’s done installing, run the following command:
Remember that RD Connection Broker .cer file? Import it now:
Import-RDWebClientBrokerCert <.cer file path>
Finally, publish the RDWebClient package to enable it:
Publish-RDWebClientPackage -Type Production -Latest
Open a HTML5 compatible browser, and navigate to https://<your rdwebaccess url>/RDWeb/Webclient/
So yes, the current version (October 6th 2018) also works on a Windows Server 2019 Remote Desktop Services deployment.
For a quick Step-By-Step guide for deploying Remote Desktop Services on Windows Server 2019, check this post: Step by Step Windows 2019 Remote Desktop Services – Using the GUI