Go to a random and simple web page, for example https://testmyiq.kandz.me/
If you open the tools for Web developers, you can just press F12 or Ctrl + Shift + I You will see the following
You can see the HTML code and the CSS code. You can change any CSS attributes and you can see the results on the fly. I clicked the title, and I added the color attribute to the following
The result now:
You can even see just the HTML without the CSS by deleting the line that ads the CSS file to the Web Page.
The result is too see just the HTML without the CSS code. Now you can understand how important is CSS.