Details

Avoid using ID selectors.

Bad: highly-specific styling for a single element via ID

#submit-button {
  ...
}

Good: reusable class

.submit-button {
  ...
}

While the CSS specification allows for multiple elements with the same ID to appear in a single document, in practice this is a smell. ID selectors should never be used for the purposes of styling an element, as it leads to overly specific styles that aren't easily shared with other elements.