Finalize Modern HTML Website Pages

I have an existing HTML site that’s partially built and now needs a professional polish and the remaining pages brought to life. The overall aesthetic must stay modern and clean—consistent typography, generous white space, and intuitive section hierarchy.

What I still need:
• Complete any unfinished page sections (common areas are the hero, About, Contact, and footer).
• Tidy up the current markup so it’s semantic, lightweight, and easy to maintain.
• Apply responsive CSS rules so the layout works smoothly from mobile to desktop without any framework bloat.
• Add a simple JavaScript-powered interaction where it enhances usability—smooth scrolling, menu toggle, or modal pop-ups are typical examples.
• If a basic contact or newsletter form is the best way to capture leads, wire it up with front-end validation and leave placeholders for the back-end endpoint.

Please build directly on the existing file structure; I’ll give you the Git repo or zipped source as soon as we start. Deliver finished HTML, CSS, and any JS in clear, well-commented code so I can hand it to a back-end developer or upload it myself with zero friction.