Drop IE11 support.

[trusktr]

Summary

Microsoft is about to do the same thing: https://docs.microsoft.com/en-us/lifecycle/products/internet-explorer-11

Plus, Microsoft already has "IE Mode" in Edge which it will support until 2029: https://www.microsoft.com/en-us/edge/business/ie-mode

There will be no reason to keep IE support soon.

What kind of change does this PR introduce?

Docs

For any code change,

• Related documentation has been updated if needed
• Related tests have been updated or tests have been added

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

Related issue, if any:

closes #1221
closes #707

Tested in the following browsers:

• Chrome
• Firefox
• Safari
• Edge
• IE 😄

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[jhildenbiddle]

As much as I appreciate the sentiment (we're all ready to put IE11 behind us), dropping support for IE11 is not as easy as removing a line item from our README.md.

We have discussed and agreed to a plan for dropping support for IE11 in the past (e.g., #657 (comment)). The TL;DR is that we can do this with a major version bump (v4 => v5), but we should not publish a new major version just to drop IE11 support.

The issue here is Docsify's history of linking to CDN URLs locked to the @latest version. This means the launch of v5 will guarantee some percentage of live Docsify sites will break. There are ways we can work around this, but the point is that v5 will not be a typical release and a conscientious effort to not break live sites will be required.

Equally important, we should not be so quick to assume legacy browsers (like IE11) are not being used or supported elsewhere. The fact that MS has and will continue to support IE11 up to June 15, 2022 indicates that there is a rational for this. Here's another data point to consider: I created css-vars-ponyfill to add support for CSS custom properties (aka "CSS variables") to legacy browsers which is downloaded ~130 million times per month from one CDN (jsDelivr) and ~500 thousand times per month from npm. Here are live stats:

We can't derive the number of IE11 users from those stats, but the stats do indicate that many developers and site visitors continue to use tools designed specifically to support IE11 even into 2022 (as sad as that may be). I continue to get questions from developers to this day asking things like how to integrate the ponyfill into their app which is using the latest version of Angular.

For comparison, here are the same live stats for docsify (~7M/mo for jsDelivr, 117K/mo for npm when written):

Just to be clear, I am not proposing that we continue to prioritize IE11 support. My recent PR (#1743) was not intended to imply that; I just wanted v4's legacy support to be in better shape when the day comes that we finally move on to v5. If we want to drop IE11 support sooner than later, let's discuss if/how we can handle the bump to v5 in a non-breaking way first and proceed from there.

FYI: The linked issues above were fixed by #1743. I've marked them as resolved and closed them.

@docsifyjs/core: I vote to close this PR and drop IE11 support with the next major version. Thoughts?

[trusktr]

What if we move the a new version (whether it is 4.x, 5.x, or etc) build output to a new path, and leave the last IE-compatible version at the same path so that @latest with the current URLs will never break?

[Koooooo-7]

I agree with @jhildenbiddle that we need not drop this IE support for now.
If user has new request on IE 11 we could mention them we will remove IE support in the future.

[jhildenbiddle]

@trusktr: What if we move the a new version (whether it is 4.x, 5.x, or etc) build output to a new path, and leave the last IE-compatible version at the same path so that @latest with the current URLs will never break?

Yep. That's an option. Users would lose the ability to load the latest version of Docsify using the @latest URL version lock or by omitting a version lock from the URL entirely, but maybe that isn't such a bad thing. We can watch the traffic to v4 using jsDelivr stats, and if/when the traffic gets low enough we can reclaim @latest from v4 and use it as intended. Could be as easy as just building to dist instead of lib for v5.

But for now...

@jhildenbiddle: The TL;DR is that we can do this with a major version bump (v4 => v5), but we should not publish a new major version just to drop IE11 support.

Let's close this PR and create a new issue where we can discuss approaches for releasing v5 (hopefully with a "how should we do it" focus instead of a "what should it contain" debate).

@trusktr -- I generally prefer to let authors close their own issues/PRs, so I'm leaving this open until you're ready.

[jhildenbiddle]

Closing due to reviewer consensus and lack of inactivity. We'll revisit legacy support in v5.