Apage layout technique that often uses a percentage value for width is called

It is called a fluid layout

A fluid web page typically is configured with percentage widths that sometimes take up 100% of the browser. They will use relative units like percentages instead of pixels and will often fill the width of the page regardless of what the width of the browser might be. They keep the same spatial weighting to all elements and works fine on different sizes of screens of similar sort. They define each area of an image layout perfectly.