Revisions to Two HTML elements with same id attribute: How bad is it really?

added 4 characters in body

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edited Oct 28, 2022 at 16:56

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Specification says UNIQUE

HTML 4.01 specification says ID must be document-wide unique.

HTML 5 specification says the same thing but in other words. It says that ID attributes must be unique amongst all the IDs in the element's tree, which is basically the document if we read the definition of it.

Avoid duplication

But since HTML renderers are very forgiving when it comes to HTML rendering they permittolerate duplicate IDs. This should be avoided if at all possible and strictly avoided when programmatically accessing elements by IDs in JavaScript. I'm not sure what getElementById function should return when several matching elements are found? Should it:

return an error?
return first matching element?
return last matching element?
return a set of matching elements?
return nothing?

But even if browsers work reliably these days, nobody can guarantee this behavior in the future since this is against specification. That's why I recommend you never duplicate IDs within the same document.

Specification says UNIQUE

HTML 4.01 specification says ID must be document-wide unique.

HTML 5 specification says the same thing but in other words. It says that ID attributes must be unique amongst all the IDs in the element's tree, which is basically the document if we read the definition of it.

Avoid duplication

But since HTML renderers are very forgiving when it comes to HTML rendering they permit duplicate IDs. This should be avoided if at all possible and strictly avoided when programmatically accessing elements by IDs in JavaScript. I'm not sure what getElementById function should return when several matching elements are found? Should it:

return an error?
return first matching element?
return last matching element?
return a set of matching elements?
return nothing?

But even if browsers work reliably these days, nobody can guarantee this behavior in the future since this is against specification. That's why I recommend you never duplicate IDs within the same document.

Specification says UNIQUE

HTML 4.01 specification says ID must be document-wide unique.

HTML 5 specification says the same thing but in other words. It says that ID attributes must be unique amongst all the IDs in the element's tree, which is basically the document if we read the definition of it.

Avoid duplication

But since HTML renderers are very forgiving when it comes to HTML rendering they tolerate duplicate IDs. This should be avoided if at all possible and strictly avoided when programmatically accessing elements by IDs in JavaScript. I'm not sure what getElementById function should return when several matching elements are found? Should it:

return an error?
return first matching element?
return last matching element?
return a set of matching elements?
return nothing?

But even if browsers work reliably these days, nobody can guarantee this behavior in the future since this is against specification. That's why I recommend you never duplicate IDs within the same document.

Fix broken tree terminology link, adjust outdated "home subtree" terminology that no longer exists in specification

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edit approved Apr 6, 2021 at 18:36

zcoop98

103
5

Specification says UNIQUE

HTML 4.01 specification says ID must be document-wide unique.

HTML 5 specification says the same thing but in other words. It says that ID attributes must be unique amongst all the IDs in itsthe element's home subtreetree, which is basically the document if we read the definition of it read the definition of it.

Avoid duplication

But since HTML renderers are very forgiving when it comes to HTML rendering they permit duplicate IDs. This should be avoided if at all possible and strictly avoided when programmatically accessing elements by IDs in JavaScript. I'm not sure what getElementById function should return when several matching elements are found? Should it:

return an error?
return first matching element?
return last matching element?
return a set of matching elements?
return nothing?

But even if browsers work reliably these days, nobody can guarantee this behavior in the future since this is against specification. That's why I recommend you never duplicate IDs within the same document.

Specification says UNIQUE

HTML 4.01 specification says ID must be document-wide unique.

HTML 5 specification says the same thing but in other words. It says that ID must be unique in its home subtree, which is basically the document if we read the definition of it.

Avoid duplication

But since HTML renderers are very forgiving when it comes to HTML rendering they permit duplicate IDs. This should be avoided if at all possible and strictly avoided when programmatically accessing elements by IDs in JavaScript. I'm not sure what getElementById function should return when several matching elements are found? Should it:

return an error?
return first matching element?
return last matching element?
return a set of matching elements?
return nothing?

But even if browsers work reliably these days, nobody can guarantee this behavior in the future since this is against specification. That's why I recommend you never duplicate IDs within the same document.

Specification says UNIQUE

HTML 4.01 specification says ID must be document-wide unique.

HTML 5 specification says the same thing but in other words. It says that ID attributes must be unique amongst all the IDs in the element's tree, which is basically the document if we read the definition of it.

Avoid duplication

But since HTML renderers are very forgiving when it comes to HTML rendering they permit duplicate IDs. This should be avoided if at all possible and strictly avoided when programmatically accessing elements by IDs in JavaScript. I'm not sure what getElementById function should return when several matching elements are found? Should it:

return an error?
return first matching element?
return last matching element?
return a set of matching elements?
return nothing?

But even if browsers work reliably these days, nobody can guarantee this behavior in the future since this is against specification. That's why I recommend you never duplicate IDs within the same document.

https://developer.mozilla.org/en-US/docs/Web/API/Document/getElementById#Notes

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edit approved Oct 19, 2016 at 12:26

Community Bot

1

Specification says UNIQUE

HTML 4.01 specification says ID must be document-wide unique.

HTML 5 specification says the same thing but in other words. It says that ID must be unique in its home subtree, which is basically the document if we read the definition of it.

Avoid duplication

But since HTML renderers are very forgiving when it comes to HTML rendering they permit duplicate IDs. This should be avoided if at all possible and strictly avoided when programmatically accessing elements by IDs in JavaScript. I'm not sure what getElementByIDgetElementById function should return when several matching elements are found? Should it:

return an error?
return first matching element?
return last matching element?
return a set of matching elements?
return nothing?

But even if browsers work reliably these days, nobody can guarantee this behavior in the future since this is against specification. That's why I recommend you never duplicate IDs within the same document.

Specification says UNIQUE

HTML 4.01 specification says ID must be document-wide unique.

HTML 5 specification says the same thing but in other words. It says that ID must be unique in its home subtree, which is basically the document if we read the definition of it.

Avoid duplication

But since HTML renderers are very forgiving when it comes to HTML rendering they permit duplicate IDs. This should be avoided if at all possible and strictly avoided when programmatically accessing elements by IDs in JavaScript. I'm not sure what getElementByID function should return when several matching elements are found? Should it:

return an error?
return first matching element?
return last matching element?
return a set of matching elements?
return nothing?

But even if browsers work reliably these days, nobody can guarantee this behavior in the future since this is against specification. That's why I recommend you never duplicate IDs within the same document.

Specification says UNIQUE

HTML 4.01 specification says ID must be document-wide unique.

HTML 5 specification says the same thing but in other words. It says that ID must be unique in its home subtree, which is basically the document if we read the definition of it.

Avoid duplication

But since HTML renderers are very forgiving when it comes to HTML rendering they permit duplicate IDs. This should be avoided if at all possible and strictly avoided when programmatically accessing elements by IDs in JavaScript. I'm not sure what getElementById function should return when several matching elements are found? Should it:

return an error?
return first matching element?
return last matching element?
return a set of matching elements?
return nothing?

But even if browsers work reliably these days, nobody can guarantee this behavior in the future since this is against specification. That's why I recommend you never duplicate IDs within the same document.

fixed dead link and some spelling

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edit approved Sep 9, 2015 at 23:00

Yay295

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added 19 characters in body

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edited Dec 28, 2011 at 12:48

Robert Koritnik

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added 23 characters in body

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edited Dec 27, 2011 at 21:04

Robert Koritnik

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added 109 characters in body

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edited Dec 27, 2011 at 20:26

Robert Koritnik

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added 333 characters in body

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edited Dec 27, 2011 at 18:58

Robert Koritnik

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answered Dec 27, 2011 at 17:29

Robert Koritnik

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Specification says UNIQUE

Avoid duplication

Specification says UNIQUE

Avoid duplication

Specification says UNIQUE

Avoid duplication

Specification says UNIQUE

Avoid duplication

Specification says UNIQUE

Avoid duplication

Specification says UNIQUE

Avoid duplication

Specification says UNIQUE

Avoid duplication

Specification says UNIQUE

Avoid duplication

Specification says UNIQUE

Avoid duplication