Answers for "how the html height can be smaller than total page size height"

css height property

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

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

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

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

css size

#myID {
  height: 100px; /* Sets the height */
  width: 200px; /* Sets the width */
}
/* or... */
.myClass {
  height: 220px; /* Sets the height */
  width: 40px; /* Sets the width */
}
/* They both work! */