Iframe generator — build the embed code, then see if it works

Almost always because the site you are embedding refuses to be embedded. Two headers control this: X-Frame-Options (with DENY or SAMEORIGIN) and the newer frame-ancestors directive in a Content-Security-Policy. If either excludes your page, the browser blocks the frame and logs the reason in the console instead of showing anything. It is not something you can override from your side — the permission belongs to the other site, so the options are to ask them, use an embed URL they publish for the purpose, or host the content somewhere that allows it.

Percentage for anything that sits inside a page you do not fully control, which is most cases. A fixed pixel width wider than a phone screen forces the whole page to scroll sideways, and that is the single most common iframe bug on mobile. The "Scale with container" option goes further: it wraps the frame in a box that keeps your chosen aspect ratio at any width, which is what you want for anything video-shaped or dashboard-shaped.

It is the accessible name of the frame. A screen reader announces an iframe as a separate region of the page, and without a title it is announced as an unlabelled frame, which tells the listener nothing about whether to enter it. It costs one attribute and it is an accessibility failure to omit it, so the generator always includes one and falls back to a generic label if you leave it blank.

Yes, and it is on by default here. It tells the browser to skip fetching the framed page until the reader scrolls near it, which matters because an iframe loads a whole second page — scripts, styles, images and all. For a frame below the fold that is a large saving for a reader who never scrolls that far. Turn it off only for a frame at the top of the page, where you want it loading immediately.

Yes — published pages set frame-ancestors * on purpose, so any site can embed them and there is no allowlist to get added to. That is the deliberate difference from the tools in the answer above: if you need something embeddable and the original refuses, publishing it here gives you a URL that will render inside your frame. The publish button below produces a page that already contains your embed, if what you want is a shareable link rather than a snippet.