Elements and attributes from ammonia’s default whitelist, links and images only with safe
destinations (is_safe_url). Scripts, styles, event handlers, frames, forms
and javascript: addresses don’t get through. Elements outside the whitelist lose just
their tags; <script> and <style> go away together with their content.
Whether the address is relative (/page, #note, image.png) or has one of the schemes
ammonia accepts by default (https:, mailto:…) — so never javascript:, vbscript:,
data: or file:. The scheme is read the way browsers read it: without spaces and control
characters around the address, without tabs and newlines inside (java\tscript:)
and regardless of case.