In HTML, an id attribute should not start with a number when used in combination with CSS or JavaScript, even though technically the HTML specification allows it. The reason is that CSS selectors and JavaScript methods (like querySelector) follow more strict rules based on CSS syntax, which doesn't allow IDs to begin with digits unless properly escaped.
Why It Matters?
If you use an ID that begins with a number (for example, id="557win"), it may function correctly in HTML by itself. However, referencing that ID in CSS or JavaScript can become problematic without special escaping. This can result in unexpected behavior or bugs that are difficult to troubleshoot.
Example (Incorrect Use)
<!-- This might work in HTML, but causes issues in CSS/JS -->
<div id="557win">Hello</div>
<style>
/* This will not work correctly */
#557win {
color: red;
}
</style>
Correct Practice
Start the ID with a letter (A-Z or a-z), followed by letters, digits, hyphens (-), or underscores (_).
<!-- Correct ID naming -->
<div id="win557 ">Hello</div>
<style>
#win557 {
color: red;
}
</style>
In Conclusion of HTML ID Start With a Number
To avoid compatibility issues with CSS and JavaScript, always start HTML IDs with a letter, not a number.