Support embedding `<iframes>` in GitLab Flavored Markdown, enabling rich content embeds from services like YouTube, Figma, and others. ### Done - [x] Ability to configure permitting iframe embeds in Markdown on an instance level, with an allowlist approach for domains permitted inside such an iframe. - [x] Embed via `` syntax, where `url` is the iframe source. - [x] Allow applying `width`/`height` parameters as these embeds by default are unhelpfully small — https://gitlab.com/gitlab-org/gitlab/-/merge_requests/224624+s - [x] Refactor to support future work — https://gitlab.com/gitlab-org/gitlab/-/merge_requests/224625+s - [x] Support transforming `<iframe>` as written or copy-pasted in so we can take embed code from YouTube or Figma — https://gitlab.com/gitlab-org/gitlab/-/merge_requests/224626+s - [x] Support default URL transformations for YouTube/Figma, so e.g. embedding a YouTube "view" URL also Just Works™ (DWIM principle) — https://gitlab.com/gitlab-org/gitlab/-/merge_requests/223870+s ### To-do for GA - [ ] Refactor of RTE resizable embedded content — https://gitlab.com/gitlab-org/gitlab/-/merge_requests/231411+s - [ ] Rich-text editor support for `<iframe>` embeds — https://gitlab.com/gitlab-org/gitlab/-/merge_requests/231412+s - [ ] Allow YouTube embeds to be opened in new windows — https://gitlab.com/gitlab-org/gitlab/-/merge_requests/231827+s - [ ] Document (https://gitlab.com/gitlab-org/gitlab/-/work_items/597027+) ### Related issues - The original customer request for embedding GitLab pipeline artifacts in an iframe (via @jcarrano) is tracked separately in #590071+.