Interstitial

An interstitial is a screen the reader did not ask for, standing between the thing they clicked and the thing they wanted. The word covers a wide family: the advert between two article pages, the app install screen a mobile site shows on arrival, the age gate, the “your download will begin shortly” page, and the warning a government service puts in front of a question people must not answer casually. What they share is the shape. The whole viewport is given over to one message, the journey is paused rather than branched, and there is exactly one way onward.

The useful distinction is against a modal dialog. A modal is an overlay on the page you are already on: the page stays behind it, escape closes it, focus is trapped and then handed back, and nothing about your position changes. An interstitial is a page in its own right. It usually has its own address, it appears in browser history, and there is nothing behind it to return to. That difference is not decorative, it decides the design. An interstitial has to say where the reader was going and carry them the rest of the way, because dismissing it is not an option the way closing a dialog is, and the browser Back button has to lead somewhere sane rather than bouncing the reader into the same screen again.

There is a genuinely good version. The GOV.UK Design System calls it an interruption page and uses it exactly once per journey, for a warning that must not be skimmed: a declaration, a consequence, a change that cannot be undone. The rules that make it work are worth copying anywhere. One message and nothing else on the screen, no secondary decisions, no navigation, plain language, and a single obvious control that continues the thing the reader started. If a page like this is being reached for twice in one flow, the flow is the problem.

The abusive version is the one most people picture, and it has real costs. Interstitial adverts that cover an article on arrival, app install screens that hide the content they promised, and countdown gated “skip” buttons all charge the reader for something they already chose. Search engines treat this as a quality problem rather than a taste one: Google has demoted pages that hide their content behind intrusive interstitials on mobile since 2017, with carve outs for legal obligations such as cookie consent and age verification, and for small banners that leave the page readable. The pattern that survives that test is the one that was serving the reader’s journey rather than interrupting it, which is also, usually, the one that would survive being explained to them.