The HTML sizes attribute is used in conjunction with the srcset attribute to specify a set of image sources for different display or device conditions for the <img>, <source>, and <link rel="icon"> elements. The sizes attribute informs the browser about the size of the image for various breakpoints, allowing it to select the most appropriate image source from the srcset based on the current viewing context. This is particularly useful for responsive web design, where the display size of an image might change based on the screen size, resolution, or other factors.

The value of the sizes attribute is a comma-separated list of source size values. Each source size value consists of:

1. A media condition (e.g., a media query) that describes a particular scenario, such as a certain viewport width. This part is optional; if omitted, the size value applies to all viewport widths.
2. A source size value that specifies the width of the slot the image will fill when the preceding media condition is true. This value can be in pixels (e.g., 300px) or as a relative width (e.g., 100vw for 100% of the viewport width).

Here is a basic example of using the sizes attribute with srcset:

<img srcset="small.jpg 500w,
             medium.jpg 1000w,
             large.jpg 1500w"
     sizes="(max-width: 600px) 500px,
            (max-width: 900px) 1000px,
            1500px"
     src="medium.jpg"
     alt="Example image">

In this example:

• When the viewport width is 600px or less, the browser is advised to use the small.jpg image, which is best suited for slots up to 500px wide.
• Between 601px and 900px in width, medium.jpg is suggested, fitting slots up to 1000px wide.
• For viewports wider than 900px, large.jpg is recommended, with the slot width set at 1500px.

The sizes attribute enhances the responsiveness of web content by ensuring that images are downloaded and displayed in the most size- and bandwidth-efficient manner possible, improving both the performance and the user experience.