Tip calculator app

I have some questions:

• I still don't understand why sometimes "getElementById" works, and sometimes not. So the question is why this happen.
• I had some issues with the input manipulation, in particoular with the custom tip input. How can I set it on focus when it appears?
• Again with the custom tip input, why my program doesn't gets the input value?

Thanks for helping.

ps. I tried the 'mobile first approach' and it's a lot easier

updates:

• I figured out that the "custom button" can be replaced by an input with a placeholder="Custom"