One ID per Element: The Rule of Thumb for IDs in HTML

1. Uniqueness: IDs are meant to be unique identifiers for elements. If an element has multiple IDs, it becomes unclear which ID should be used for targeting in CSS or JavaScript.
2. Standards Compliance: As per HTML specifications, an element can only have one id attribute. Including multiple id attributes violates these specifications and can lead to unexpected behavior in different browsers.
3. Code Clarity: Having multiple IDs for an element makes the code harder to read and maintain. It's generally considered bad practice and can lead to confusion for other developers working on the code.

Here's an example to illustrate:

<p id="firstID secondID">This is a paragraph</p>

In this example, even though both firstID and secondID are assigned, only the first encountered ID, which is firstID, will be recognized by the browser. Trying to target the element using secondID in CSS or JavaScript will not work.

Related Issues and Solutions:

• Multiple Styles for an Element: Instead of using multiple IDs, you can achieve the desired styling by applying multiple class names to the element. Classes are not unique and can be applied to several elements at once.

<p class="primary secondary">This is a paragraph</p>

• Targeting Elements with Similar Characteristics: If you need to target multiple elements with similar characteristics, consider using CSS selectors that target elements based on their tag name, class name, or other attributes. For example, you can use the following selector to target all paragraphs with the class important:

p.important {
  /* Styles for important paragraphs */
}