The way browsers handle CSS styling when multiple elements have the same ID

I've been thinking about how IDs are supposed to be unique in HTML documents, but often times that rule is not followed. I'm curious to know how browsers handle CSS applications when multiple elements share the same id.

From my initial testing, it seems like browsers apply the id-specific CSS rule to all elements with the same id. This behavior appears consistent across Internet Explorer 8, Chrome, Firefox, and Opera.

I wonder if this is a deliberate choice made by browsers to accommodate sloppy HTML or if there are exceptions to this behavior. What potential unintended consequences could arise in CSS layout as a result of multiple elements sharing the same id?

htmlcss

Answer №1

When it comes to CSS display, the error recovery mechanism is to treat the CSS as a class.

What are some potential consequences in CSS layout when multiple elements share the same ID?

In terms of CSS layout, there may not be significant implications, but...

You may face difficulties targeting a specific element or overriding a CSS rule with an existing ID. In such cases, adding a unique ID higher up in the hierarchy or using inline <style> could be necessary.

Moreover, using jQuery or similar JavaScript libraries might pose challenges since they rely heavily on IDs for functionality.

IDs also serve as Fragment Identifiers - non-unique IDs can cause issues in this context.

Answer №2

If you want to see an example of unintended consequences, take a look at the question I addressed:

Another instance of IE 'Object expected' error with no information

The issue arises with this HTML code snippet, repeated for each tab:

<div class='tab' id='introduction'>
    <h2 id='introduction'>Introduction</h2>
</div>

<div class='body' id='introduction' style='display:block'>

The problem stems from having two elements with id='introduction'.

There are several reasons why this is not advisable:

  • It leads to validation errors.

    Line 37, Column 27: Duplicate ID introduction.
    Line 36, Column 39: The first instance of ID introduction was here.

  • It causes issues with your tabs in IE7.

Answer №3

Deviating from the standard rules makes this unreliable for consistent use. Testing in each browser is necessary as behavior may vary between versions.

Your approach seems logical to me; the complexities likely lie more with javascript handling rather than css in this scenario.

There should be no unforeseen issues. Just like using the same class for multiple elements, overriding styles with ids should work without any unexpected conflicts.

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