Core:Manipulation: Add basic TrustedHTML support

This ensures HTML wrapped in TrustedHTML can be used as an input to jQuery
manipulation methods in a way that doesn't violate the
`require-trusted-types-for` Content Security Policy directive.
This commit builds on previous work needed for trusted types support, including
gh-4642 and gh-4724.

One restriction is that while any TrustedHTML wrapper should work as input
for jQuery methods like `.html()` or `.append()`, for passing directly to the
`jQuery` factory the string must start with `<` and end with `>`; no trailing
or leading whitespaces are allowed. This is necessary as we cannot parse out
a part of the input for further construction; that would violate the CSP rule -
and that's what's done to HTML input not matching these constraints.

No trusted types API is used explicitly in source; the majority of the work is
ensuring we don't pass the input converted to string to APIs that would
eventually assign it to `innerHTML`. This extra cautiousness is caused by the
API being Blink-only, at least for now.

The ban on passing strings to `innerHTML` means support tests relying on such
assignments are impossible. We don't currently have such tests on the `main`
branch but we used to have many of them in the 3.x & older lines. If there's
a need to re-add such a test, we'll need an escape hatch to skip them for apps
needing CSP-enforced TrustedHTML.

See https://web.dev/trusted-types/ for more information about TrustedHTML.

Fixes gh-4409
Closes gh-4927
Ref gh-4642
Ref gh-4724

1 files changed, 7 insertions, 0 deletions

diff --git a/src/core/isObviousHtml.js b/src/core/isObviousHtml.js
new file mode 100644
index 000000000..976f81219
--- /dev/null
+++ b/src/core/isObviousHtml.js
@@ -0,0 +1,7 @@
+function isObviousHtml( input ) {
+	return input[ 0 ] === "<" &&
+		input[ input.length - 1 ] === ">" &&
+		input.length >= 3;
+}
+
+export default isObviousHtml;