Kind of like a WYSIWYG Editor but with HTML/CSS instead.

Every WYSIWYG editor is a HTML editor. You just need to make sure your group permission allow the use of HTML to edit it directly. 

In addition, you can also create Pages blocks that are purely HTML or PHP code and then add them to your site on the front end. 

@opentype - Hi. That's what I thought, but every time I enter HTML and click save and review the page, it's just code. Like, the code didn't transfer. I'm on the admin account and this still doesn't work. Is it a glitch I'm facing or something? Am I supposed to change the value of it somehow?

In the WYSIWYG editor block you first have to click the 'Source' button (if you don't see it, that's most likely because you do not have permission to post HTML), paste the HTML in, and then save.