VS Code W3C Validation extention
The extension to enable W3C validation support by the Nu Html Checker library.
This extention works using Java applications. You must have Java 8 or later on your local environment. If you don't have them installed, download and install a latest Java Development Kit (latest Java 8 is the minimum requirement).
After installing (or installed) the JDK, you would need to configure the Java execution path for your environment. Please restart the VS Code in order to load reliably the changed environment variables.
Then open your HTML document(s).
Setting the JDK
The path to the JDK requires to be set in the environment.