Need mockups converted to HTML

This is a standard request. I have mockups being created by the design team, and they need to be converted to modern HTML5/CSS3, to be used in web-based frontends. Some are mobile, some are responsive desktop. We are using Angular & Ionic to build hybrid apps, however they also get build as PWA’s for the web. This request is for only converting the mockups to HTML. In your response, please answer the following: [1.] what’s 12 – 3/11? [2.] How do I centrally (horizontally & vertically) position a square using flex? [3.] What are your preferred frontend frameworks? How do you write HTML/CSS code that is responsive, looks good on both mobile and desktop? Thanks. I look forward to reading your responses.