Tried googling this, but it's been SEO/content marketed into oblivion so I don't even know where to ask this question.
Otherwise, freeCodeCamp's (New) Responsive Web Design certificate will make you comfortable with the syntax and DOM manipulation.
Also, I wrote a "How Web Apps Work" blog series that was specifically targeted at folks coming from a Java or C++ background. It's an extensive overview of web dev terms and concepts, covering HTTP, client-side development, HTML/CSS/DOM, JS, and API calls:
- https://blog.isquaredsoftware.com/series/how-web-apps-work/