Allows sanitized URLs for CSS properties. These can be abused for information
leakage, but only if the CSS rules are already set up to allow for it. That is,
an attacker cannot cause information leakage without controlling the style rules
present, or a very particular setup.
Fixes#8514.
This is based on Angular 1's implementation, parsing an HTML document
into an inert DOM Document implementation, and then serializing only
specifically whitelisted elements.
It currently does not support SVG sanitization, all SVG elements are
rejected.
If available, the sanitizer uses the `<template>` HTML element as an
inert container.
Sanitization works client and server-side.
Reviewers: rjamet, tbosch , molnarg , koto
Differential Revision: https://reviews.angular.io/D108
