Thanks for including a shot of your Sketch mockup; when I learned HTML/CSS/JS, it was 2014, and the "wireframe to HTML" craze was in full swing, and discouraged the creation of mockups.
Having built some sites since then, I cannot say enough about the value of using something like Sketch or Figma to design the site first. This means creating the style guide (typographic scale, primary/secondary colours, layout), mocking up some pages, and figuring out if you're happy with it. Only then do I start writing the HTML and CSS.
Before that, I'd code everything first and always feel like the design was off, but I didn't know why. Building it out in a prototyping tool helps you visualize the site much better, and when it's time to code, you will already have the color codes, font sizes, line-heights etc. ready to paste into the CSS.