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Comment by robin_reala | Hacker News Reader

It’s Postel’s law at the end of the day: “be conservative in what you do, be liberal in what you accept from others”. As a site owner I want my site to fail loudly and quickly before a user sees a problem; as a user I never want to see a problem.

ePub is in a nice place: the number of documents to check for errors is reasonable, and the resulting artefect is designed to be shipped and never (or rarely) amended. That means that we can shift the balance towards strict parsing. But for a web site of thousands (or millions) of documents that are being amended regularly, the balance shifts back to loose parsing as the best way of meeting user needs.

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Postel's Law sounds nice but it can result in major problems. It results in a de facto spec that differs from the written spec, and disagreements about what a piece of data actually means can lead to bugs and even security vulnerabilities.

Having strictly parsed HTML from the start would be fine. You'd check it before you ship it and you'd make sure it's valid.

Requiring it now would be a disaster, of course. There's so much malformed HTML out there. But making HTML parsers accept garbage at the beginning was the wrong choice.

The widespread acceptance of Postel’s Law also encourages poor authorship, because if you know clients have to be liberal in what they accept, there is no incentive to be conservative in what you send.

Isn't the developer always the first user? With strict parsing, testing a site before launch would show you the problem right there and allow you to fix it to launch a bug free site.

What about a 5 year old client hitting a new server or the reverse? Is the only solution just don't do that?

That is what the HTML 4 !doctype Tag - https://www.quackit.com/html/html_4/tags/doctype_html_public... - was meant for. Ideally, an old client that cannot parse a newer HTML spec should inform the user to update the client. Developers could then either support old versions of their site (more easily, in such a case) or can simply decide not to deal with outdated browsers. (In fact, isn't that what most BiGTech sites actually do now, through convoluted browser feature detections - "Your browser is no longer supported ...").

Postel's law has turned out to be good for prototypes and experiments, but a horrible way to make anything with any degree of reliability.

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