width in css

/* <length> values */
width: 300px;
width: 25em;

/* <percentage> value */
width: 75%;

/* Keyword values */
width: max-content;
width: min-content;
width: fit-content(20em);
width: auto;

/* Global values */
width: inherit;
width: initial;
width: unset;

html {
    position: relative;
    min-width: 1024px;
    min-height: 768px;
    height: 100%;
}
<!-- Remove height 100% if needed fixed height -->

<div style="background-color:navy;">
  <div style="width:50%; margin-left:20%; background-color:chartreuse;">
    Width: 50%, Left margin: 20%
  </div>
  <div style="width:30%; margin-left:60%; background-color:pink;">
    Width: 30%, Left margin: 10%
  </div>
</div>