On Mon, Feb 05, 2018 at 09:26:14PM -0700, Samuel Wales wrote:
> 
>   [[http://whatever.com]]
> 
> i do not know the standard, but i think that some email clients will
> not linkify this because of the brackets.

Correct.

> are we sure that [] are canonical?  not <>?  is it the clients that
> are wrong?

No, it's because the square brackets are valid characters for use in
file names and thus can be part of the URL itself.  There's no way for
a MUA to recognise the difference.

You can see this demonstrated here:

http://www.adversary.org/test/]

And here:

http://www.adversary.org/test/]]

Angle brackets, however, are not valid in URLs and recognisably used
within HTML, XHTML, XML and variants.  Since most MUAs achieve their
WYSIWIG interfaces via some means of implementing HTML (or a
bastardisation of it), the angle brackets are recognised as a means of
specifying a particular type of object.  Some MUAs will leverage the
mailcap file to be more specific with this.  Some, of course, won't
and some wander off on strange bastardised rich text tangents or
custom protocols (insert pointed look at Microsoft Outlook here).

Mostly, though, GUI MUAs went for HTML 4.01 and/or an XHTML variant
after Netscape Communicator hit the Net in the late '90s and everyone
(eventually) followed suit.  Along with retaining some simplistic
carry over from plain text emails similar to basic text mark-up in
org-mode (e.g. *bold*, /italics/, _underline_ and so on).  Most of
which were de facto standards, not official ones.  Basically the geek
equivalent of demonstrating how custom and tradition were more
pervasive than law (RFCs and MUA design).


Regards,
Ben