Greetings,
First, thank you for providing wildwebdeveloper to the Eclipse community.
The issue is simple: The usage of private properties in JS classes leads to error markers in valid JS code. Below, I provide the version of wildwebdeveloper I use - which appears to be the latest one - and two screenshots of a JS-file opened in the GenericEditor. The first screenshot shows valid code without a private field, the second one valid code with a private field and a lot of error markers. I could reproduce the issue in multiple files and workspaces.
Documentation about private properties at MDN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_properties
Versions:

Valid code without a private property:

Valid code with a private property:

Kind regards