HTML Classes and IDs Problems related to duplicated IDs
Example
Having more than one element with the same ID is a hard to troubleshoot problem. The HTML parser will usually try to render the page in any case. Usually no error occurs. But the pace could end up in a mis-behaving web page.
In this example:
<div id="aDiv">a</div>
<div id="aDiv">b</div>
CSS selectors still work
#aDiv {
color: red;
}
But JavaScript fails to handle both elements:
var html = document.getElementById("aDiv").innerHTML;
In this casehtml variable bears only the first div content ("a").
