Intro section using HTML5 & Flexbox

I'm a little rusty, it's been a while, and I'm trying to get back to it. I put the code through the w3 html and css validator, so on that front it should be ok. When I pressed 'tab' it went through the register button and the social media icons.

One question I had is if using <a> vs <button> is better in this example.

If there's any general feedback after looking at the site and/or the code, especially with layout and spacing, I would appreciate it.

Thank you and Cheers✌️✌️