Further complimenting the other two answers, you can even put the css in a separate file and include a css argument in your yaml !!

in your quarto file:

---
title: "How to justify text in Quarto"
format: html
engine: knitr
css: main.css
---

## Quarto

Quarto enables you to weave together content and executable code into a finished document. To learn more about Quarto see <https://quarto.org>.

Quarto is based on Pandoc and uses its variation of markdown as its underlying document syntax. Pandoc markdown is an extended and slightly revised version of John Gruber's Markdown syntax and Markdown is a plain text format.

Markdown is a plain text format that is designed to be easy to write, and, even more importantly, easy to read:

Markdown is a plain text format that is designed to be easy to write, and, even more importantly, easy to read:

then in your main.css (or whatever you want to call it)

p {
  text-align: justify
}