An iframe (inline frame) is an HTML element that embeds one webpage inside another. The embedded content loads in its own separate browsing context, with its own document, its own styles, and its own scripts. From the outer page's perspective, the iframe is a single embedded object.
Common uses: embedded maps, payment forms from external processors, contact forms from third-party providers, video players from YouTube or Vimeo, and widgets from services like Calendly or Typeform. The content you see inside the iframe is actually a separate page being rendered within that rectangular area.
The separation matters when something inside an embedded form or widget looks wrong. Your site's CSS can't reach inside an iframe that loads content from a different domain. Browser security restrictions prevent it. Whatever is controlling the styling of that embedded content is controlled by whoever hosts it, not by you. Loupely Lens can read CSS from iframes that are same-domain (on the same site), but not from cross-origin iframes. When you click an element inside a cross-origin iframe, Lens will surface this limitation rather than attempting a capture that would return incomplete data.