For good practices:
- Avoid doing inline css. Keep your styles in a separate file. So that the code will look cleaner. You can use SASS or Styled Components. There are also bunch of style component libraries. I assume that you are solving exercise to make your fundamentals strong. Then I prefer Styled Components with react.
- In visual case, enlarge the size of your calculator. Its tough to read small texts. Keep a min-width of the container.
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