@aphillips @stasm Would you be ok with this landing as "Proposed"?

I’ve been thinking about an alternative design for this. I’m going to take note of it here briefly and add a new section in the doc later this week. I'm also fine with this PR landing as "Proposed" before I edit it -- I'll open a new PR then. (Actually, that may be preferable.)

Also, please be gentle, it’s a work-in-progress which I’m sharing early to see if there’s any support for it.

My goal is, as usual, to avoid adding new sigils to the syntax. Instead, I’d like to leverage the fact that we already have the concept of keywords in the syntax. Furthermore, with #469, we were able to find a good solution by adding a new keyword (input). I think we can try to do the same here.

I’d like to propose we add two new declaration types called openclose and standalone, along with new placeholder syntax: {foo}, {/foo}, and {foo/}. I've come to think that the only good syntax for spannables is one that looks like a markup language. There's also vast prior art in using some sort of [foo] and [/foo] in various web forums. Note that this would require dropping unquoted literals as operands, which I think is an acceptable trade-off.

{{
    openclose {strong :html class=foo onclick=bar}
    standalone {img :html src=http://…}
    {{This is {strong}bold{/strong} and this is {img alt=|an image|/}.}}
}}

The declarations are optional (similar to input). If they’re absent, then any {foo} is interpreted as a span-open, any {/foo} as a span-close, and {foo/} as a standalone element. If declarations are present, they specify the kinds of markup precisely, making {strong/} and {img} and {/img} invalid in the example above. Additionally, they allow to add non-localizable attributes to makrup.

I’m not yet sure how to allow more than one element of the same name but with different attributes. Probably something like:

standalone img1 = {img :html src=http://a}
standalone img2 = {img :html src=http://b}

I opened #513 which adds the above idea as an alternative to the design doc.