Adds clarifying note to using sandbox with iframe tag (mdn#31164)

* Add clarifying note to using sandbox with iframe tag

* Clarifying language around sandbox attribute pitfalls

---------

Co-authored-by: Chris Mills <chrisdavidmills@gmail.com>

files/en-us/web/html/element/iframe/index.md

@@ -104,7 +104,7 @@ This element includes the [global attributes](/en-US/docs/Web/HTML/Global_attrib
  - `allow-popups`
  - : Allows popups (like from {{domxref("Window.open()")}}, `target="_blank"`, {{domxref("Window.showModalDialog()")}}). If this keyword is not used, that functionality will silently fail.
  - `allow-popups-to-escape-sandbox`
- - : Allows a sandboxed document to open new windows without forcing the sandboxing flags upon them. This will allow, for example, a third-party advertisement to be safely sandboxed without forcing the same restrictions upon the page the ad links to.
+ - : Allows a sandboxed document to open a new browsing context without forcing the sandboxing flags upon it. This will allow, for example, a third-party advertisement to be safely sandboxed without forcing the same restrictions upon the page the ad links to. If this flag is not included, a redirected page, popup window, or new tab will be subject to the same sandbox restrictions as the originating `<iframe>`.
  - `allow-presentation`
  - : Allows embedders to have control over whether an iframe can start a [presentation session](/en-US/docs/Web/API/PresentationRequest).
  - `allow-same-origin`
@@ -125,6 +125,8 @@ This element includes the [global attributes](/en-US/docs/Web/HTML/Global_attrib
  > - When the embedded document has the same origin as the embedding page, it is **strongly discouraged** to use both `allow-scripts` and `allow-same-origin`, as that lets the embedded document remove the `sandbox` attribute — making it no more secure than not using the `sandbox` attribute at all.
  > - Sandboxing is useless if the attacker can display content outside a sandboxed `iframe` — such as if the viewer opens the frame in a new tab. Such content should be also served from a _separate origin_ to limit potential damage.
 
+ > **Note:** When redirecting the user, opening a popup window, or opening a new tab from an embedded page within an `<iframe>` with the `sandbox` attribute, the new browsing context is subject to the same `sandbox` restrictions. This can create issues — for example, if a page embedded within an `<iframe>` without a `sandbox="allow-forms"` or `sandbox="allow-popups-to-escape-sandbox"` attribute set on it opens a new site in a separate tab, form submission in that new browsing context will silently fail.
+
 - `src`
  - : The URL of the page to embed. Use a value of `about:blank` to embed an empty page that conforms to the [same-origin policy](/en-US/docs/Web/Security/Same-origin_policy#inherited_origins). Also note that programmatically removing an `<iframe>`'s src attribute (e.g. via {{domxref("Element.removeAttribute()")}}) causes `about:blank` to be loaded in the frame in Firefox (from version 65), Chromium-based browsers, and Safari/iOS.
 - `srcdoc`