What is the method for adjusting the width of an HTML/CSS element to match the size of a mobile screen?

edit

The method for adjusting the width of an HTML/CSS element to match the size of a mobile screen is to use responsive web design techniques, such as using media queries and flexible layout grids. Media queries are used to define different styles based on the size of the viewport or device screen, and flexible layout grids use percentage-based widths instead of fixed pixel values to allow elements to adapt to different screen sizes. Additionally, using relative units like em or rem instead of fixed pixel values can also help ensure that an element's width adjusts proportionally to the size of the screen.