Tested this issue on Ubuntu 14.04 using chrome latest stable #58.0.3029.110 and beta #59.0.3071.71. By pasting the step-1 in console observed a popup displaying 1 with 2 different tittles as shown in the below screen shot.

Gareth@ Could you please confirm which is the expected behavior of the issue from the below screen-shot? Checked the same on Firefox observed no title in the popup.

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Hi @brajkumar the behaviour has nothing to do with the title. The character is being treated as whitespace the alert shouldn't be called according to the specification. Both Firefox and Chrome don't follow the specification and treat it as whitespace. I reported this to Mozilla too and they mentioned it's because of backwards compatibility that they treat this character as whitespace. See the following comment:
http://searchfox.org/mozilla-central/rev/a14524a72de6f4ff738a5e784970f0730cea03d8/js/src/vm/Unicode.h#245-247

So I guess Chrome also does this for backcompat.

Thank you for providing more feedback. Adding requester "brajkumar@chromium.org" to the cc list and removing "Needs-Feedback" label.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

As per comment #2 confirming this is not a chrome specific issue, so marking it as untriage for more updates on this issue.

Thanks!

1, As I understand this, the reporter is correct and we are not spec compliant.
2, This is trivial to fix.
   (Fix + regression test in https://chromium-review.googlesource.com/530849)
3, This is yet another deliberate spec deviation for cross-browser compatibility.

I'll look into web compatibility before deciding how to proceed.

Firefox: Same behaviour as Chrome.

---------------------

Our source comments indicate this was done for Firefox compatibility.

We ensure the current behaviour via our import of the Mozilla test suite (mozilla/ecma_3/extensions/regress-368516). Observe the 'ecma_3' path segment.

This looks to be based on https://bugzilla.mozilla.org/show_bug.cgi?id=368516, which is an 11-year-old issue about breakage of a then-current beta of Yahoo! Mail and .mac web galleries (Apple's then-current online thingy). This was fixed (i.e., current behaviour was introduced) in Mozilla ~10 years ago.

MS Edge: Syntax error (invalid character)

Web compat (per Dan's comment on the CL):
- Firefox: accepts (same as Chrome)
- Edge: syntax error (invalid character)
- Safari: rejects it [probably also syntax error]

I'd say this is low risk for the web. Dan raises the issue of potential node.js incompatibilities.

I agree that with the browsers split this is likely safe to ship in Chrome. As for the Node worry, I don't think there's much to be done there: we can't add a usecounter, and I don't think we should let Node's till-now-V8-specificness hold us back from web/spec interop.

TL;DR: I think this is a good change to make.

Fix should appear in V8 6.1 / Chrome M61.

If compatibility issues are reported, we'll likely roll this back, though.