I think your knowledge of CSS might be outdated. What's wrong with the following CSS snippet for example?

    body {
      max-width: 70ch; /* Limits the text column to a comfortable reading width */
      margin: 0 auto;  /* Centers the text column on large screens */
      padding: 1.5rem; /* Prevents text from touching the edges on small/mobile screens */
      line-height: 1.6; /* Gives the lines room to breathe */
    }

As for font size, you can use

    font-size: 1rem;

to respect the browser's default font size.