How do I make a full-bleed banner section on Squarespace?

Squarespace 7.1 sections have a max-width content container by default that prevents true full-bleed layouts. You need to override both the section padding and the inner content wrapper width.

Target the section by its data-section-id and remove all constraints so the background and content stretch from edge to edge across the viewport.

[data-section-id="YOUR_SECTION_ID"] {
  padding: 0 !important;
}
[data-section-id="YOUR_SECTION_ID"] .content-wrapper {
  max-width: 100% !important;
  padding: 0 !important;
  width: 100vw;
  margin-left: calc(-50vw + 50%);
}
[data-section-id="YOUR_SECTION_ID"] .section-background {
  position: absolute;
  inset: 0;
}