// The frozen base URL must be immediately set for an element whenever any of the following situations occur:
// - The base element is the first base element in tree order with an href content attribute in its Document, and its href content attribute is changed.
// 2. Let urlRecord be the result of parsing the value of element's href content attribute with document's fallback base URL, and document's character encoding. (Thus, the base element isn't affected by itself.)
// 3. Set element's frozen base URL to document's fallback base URL, if urlRecord is failure or running Is base allowed for Document? on the resulting URL record and document returns "Blocked", and to urlRecord otherwise.
// FIXME: Apply "Is base allowed for Document?" CSP