CODEWITHSUNDEEP
htmlsyntaxtagsquirks-modeerror-recovery
JoeL
JoeL

Reputation: 710

Why does HTML with poor and improper tag usage still work?

The tl;dr version is why does html that doesn't close tags properly still work error free?

I'm learning more and more HTML each day but I'm still quite a beginner. So I don't understand why badly written HTML without properly closing tags still works. I was using an email template for a task at work and was curious about the HTML behind it so I loaded to code into an editor.

I came across 9 separate tags that don't close: <td> <center> <div> <p> <div> <td> <tr> tbody> <table> The code starts with an <html> tag as it should, but in the <body> after a <table> and <tr>, it starts another <html> <head>... etc set of tags. In the two <style> tags, they both say say the same thing and have an extra closing brace li { margin-bottom: 10px; } }.

When I load just this code into my browser the page still visually appears how it is supposed to. In Firebug though, after the first body tag, it skips table, tr, html, head, and body, and goes straight to just showing the first <div>.

Why is it that a webpage (because I'm sure this must be a somewhat common thing out there) that has missing closure tags, extra <html> and <body> tags, etc still able to function properly?

Upvotes: 1

Views: 458

Answers (2)

xxbbcc
xxbbcc

Reputation: 17347

Misformed HTML does not work correctly - browsers try to guess the intent of the HTML structure and display whatever the result of the guess is. This is the result of an unfortunate decision to allow poorly-formed HTML to display, rather than rejecting it and forcing the author to fix such problems.

When you see misformed HTML looking correct on the screen, it's not the result of correct behavior: it's the result of a lucky guess on the part of the browser (obviously, a tiny problem is easier to fix by guessing than a massive structural problem that spans the entire HTML structure).

It comes down to treating HTML as content (which it is not), rather than a formal language (which it is): content authors were (and are) considered non-technical people and forcing them to fix problems with "content" was seen too hard a requirement.

Upvotes: 1

recursive
recursive

Reputation: 86124

I think this is an application of the Robustness Principle.

Be conservative in what you do, be liberal in what you accept from others

I'd argue this is an inevitable outcome in a landscape of competing browsers. If an HTML error prevents a site from working in browser A, but browser B is able to guess a correction, users will tend to use browser B, as A appears to be broken. This has been going on since Netscape 3 or earlier.

Upvotes: 1

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