☂️ Handle more content features for beta

[gnprice]

This is a subset of:

• Systematically survey message content for unimplemented features #190

For the beta, we won't implement absolutely every feature that appears in the content of Zulip messages. (For example, we'll likely omit TeX #46, and most of polls #164.) But there are a number of features that will be straightforward to implement and we should just do.

Specifically:

• Handle headings <h1>…<h5>, as well as <h6> #192
• Handle <hr> horizontal lines #353
• Handle strikethrough, the <del> element #357
• Handle <time> elements ("global times") #354
• Blank text nodes after code blocks are "unimplemented" #355
• Handle spoiler blocks #358
• Give a rough preview of TeX content #359
• Handle video previews #356
• perhaps Show image previews side-by-side when consecutive #193

[gnprice]

I just scrolled through the past week of public chat.zulip.org traffic looking for unimplemented content. I found five types of it: #192 was already in the tracker, and I filed #353, #354, #355, and #356. Also filed #357, #358, and #359 for issues that didn't come up in the past week's traffic there but were already noted in the description above.

I think that probably covers what we want for the beta. If we find something else, we can of course add it.

I removed the general #194 from this list, because I think if we cover all of the above — including #359 which is sort of a specialized version of #194 — then it's fine in the beta to have the development-oriented red "unimplemented" text if someone does run into something we don't support.

[gnprice]

All the issues in the above list are now complete! The last was #356 just now. Therefore closing this umbrella issue.

There are more content features to implement, and we'll continue plowing through them (as well as filing more). They can all be found under the "a-content" label, and in particular one can search for issues with that label in the current Summer 2024 milestone. At the moment 9 of those are open.