Responsive tip calculator app

• Should I include a <form> element? I didn't put in one, because the page doesn't submit any data from the inputs.

• Does the "Select Tip %" has to be a <label>. If yes, what should I give for it's for attribute.

• The input fields are text type. I originally wanted to use number type, but then the spin buttons show up. When I searched for a way to remove it, the only way I found was a non-standard one(::-webkit-outer-spin-button pseudo-element). Is there a proper way to do it?

• I gave inputmode="numeric" for the input fields, since I couldn't set them to type="number"(because of the spin buttons), but the w3c validator gives a warning since it's not supported by every browser. Is there a better way for this whole number type input field?

• It's a broad question, but is there any problem with my script? Until now I only wrote 10-20 line scripts with simply one or two event listeners (I tried to provide proper description with comments).

Thank you for the feedback.