Tip Calculator App using Flexbox and CSS Grid

I managed to make the features work, but I still don't understand how to make the code HTML Semantics. I'm also not sure if there's a better way to position the "SPLITTER" text and to make number of people input form border red as I put 0 number (it only turns red after I click somewhere outside the form, I think it's overridden by input form focus default of the form control). Any suggestions are definitely welcome