Why Do Duplicate IDs in HTML Sometimes Work, and What's the Best Practice?

<p>

<p>Multiple Elements with Identical ID Respond to CSS Selector

<p>It is generally considered poor practice to assign multiple elements the same ID within a single webpage. According to W3C documentation, ID attributes must have unique values to uniquely identify their respective elements.

<p>However, certain situations, such as using jQuery plugins, may inadvertently result in duplicate IDs. Surprisingly, browsers tend to handle this situation by "failing silently." They attempt to interpret the intent behind the invalid HTML and adjust their behavior accordingly.

<p>For instance, consider the following code:

#red {
  color: red;
}

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<p>Despite the duplicate IDs, both paragraphs will appear red in all major browsers. However, this behavior is not guaranteed and can lead to unexpected side effects.

<p>For example, accessing the element by its ID using document.getElementById('red') will only return the first element. To select both elements, you would need to use an attribute selector like document.querySelectorAll('p[id="red"]'). However, this approach is not supported in IE7 and below.

<p>To avoid potential issues, it is strongly recommended to use class names instead of IDs for targeting multiple elements with CSS. Class names are expressly designed for this purpose and ensure consistency across all browsers.

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