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Using duplicate IDs in HTML

Well today I'm being a bit controversial. Let us see what the HTML5 spec says about unique IDs in a HTML file.
The id attribute specifies its element's unique identifier (ID). The value must be unique amongst all the IDs in the element's home subtree and must contain at least one character. The value must not contain any space characters. An element's unique identifier can be used for a variety of purposes, most notably as a way to link to specific parts of a document using fragment identifiers, as a way to target an element when scripting, and as a way to style a specific element from CSS. Yes its been mentioned almost everywhere on the planet that ID must be unique. Now let us look at the below code,


Launch


dup.css


#p2 {
  background-color: yellow; 
}

dup-id.html

<!DOCTYPE html>
<html>
  <head>
    <title>Duplicate ID Tester</title>
<link rel="stylesheet" href="dup.css" />
  </head>
  <body>
<p><a href=&quo…