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**Canonical tags** are essential HTML elements that help search engines understand the preferred version of a webpage. When you have multiple URLs with identical or very similar content, such as `/product.html?color=red` and `/product.html?color=blue&size=medium` both showing the same product description, a **canonical tag** tells search engines like Google which URL is the main, authoritative source. This prevents issues like **duplicate content penalties** and consolidates ranking signals, improving your **SEO performance** by ensuring search engines index the right page. Think of it as a clear instruction: This is the original.