Rewrite the "removing features" section, incorporating "Support Existing Content" from the HTTP Design Principles

[jyasskin]

Fixes #174.

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[martinthomson]

It would be good to

1. Make this a positive statement.
2. Reference other things we've said about this general thing.

[lolaodelola]

Does it matter if the user agent is popular if the change already affects most user agents?

Edit: I misread, but I think it's worth being specific about what "popular" means here

[jyasskin]

I added more detail about why it matters that the other UAs are popular: it makes it more likely that they would have received bug reports if anyone's relying on the content, and it means that if we're increasing interoperability, we're not breaking the dramatically larger number of users.

[matatk]

Is 'even more' really needed here?

Suggested change

	but prevents even more security breaches.
	but prevents security breaches.

Or perhaps

Suggested change

	but prevents even more security breaches.
	but prevents emerging security breaches.

(or something similar)?

[csarven]

@matatk , "even more" could be dropped but it seems to match with "lots of" (or "significant"). Not sure if "far more" sounds better but here it is:

Suggested change

	but prevents even more security breaches.
	but prevents far more security breaches.

[jyasskin]

I think it's important that we compare the amount of breakage with the amount of benefit. "emerging" doesn't really do that. "far more", on the other hand, might set the bar too high. I'm not actually sure that we need more instances of benefit than instances of harm, if the severities of the instances compare well, but this is just inside the SSL example, where I think it does prevent quantitatively more potential attacks. @martinthomson would know the details there. We could also pick a different example; this one was just on my mind since Martin had mentioned it recently.

The following would be fine with me, if other people prefer it:

Suggested change

	but prevents even more security breaches.
	but prevents more security breaches.

[csarven]

Suggested change

	For example, removing old SSL and TLS versions breaks lots of content
	For example, removing old SSL and TLS versions breaks significant content

[jyasskin]

"Significant" invites a value judgement of how important the content is, while I was just trying to talk about the amount. We could use "many pages", I guess? But it's also an intentional choice to be somewhat informal here: we're trying to communicate, not impress people.

[jyasskin]

I think it's important that we compare the amount of breakage with the amount of benefit. "emerging" doesn't really do that. "far more", on the other hand, might set the bar too high. I'm not actually sure that we need more instances of benefit than instances of harm, if the severities of the instances compare well, but this is just inside the SSL example, where I think it does prevent quantitatively more potential attacks. @martinthomson would know the details there. We could also pick a different example; this one was just on my mind since Martin had mentioned it recently.

The following would be fine with me, if other people prefer it:

Suggested change

	but prevents even more security breaches.
	but prevents more security breaches.

[jyasskin]

"Significant" invites a value judgement of how important the content is, while I was just trying to talk about the amount. We could use "many pages", I guess? But it's also an intentional choice to be somewhat informal here: we're trying to communicate, not impress people.

[jyasskin]

Re @martinthomson's comment, I think we might have a higher-level principle that "The web is stable", which can cover both keeping backward compatibility and ensuring some forward compatibility. I'm inclined to try to merge this change, and then add that one, but I could reverse it if folks prefer.