How do you express the pixel value of the width attribute?

To express the pixel value of the width attribute, you would typically use a numerical value followed by the unit "px" to indicate pixels. Here's an example:

1. Open the HTML file or HTML editor where you want to set the width attribute.

2. Locate the element or tag to which you want to apply the width attribute. For instance, if you want to set the width of an image, find the "img" tag.

3. Add the width attribute to the element, and assign a numerical value followed by "px". For example, if you want the width to be 300 pixels, you would write: `width="300px"`.

4. Save the changes to the HTML file and refresh/reload the page in a web browser to see the effect.

Remember, the "px" unit used in the width attribute represents pixels. Other units like percentage (%) or viewport units (vw) can also be used depending on your requirements.