responsive text css

/* Uses vh and vm with calc */
@media screen and (min-width: 25em){
  html { font-size: calc( 16px + (24 - 16) * (100vw - 400px) / (800 - 400) ); }
}

/* Safari <8 and IE <11 */
@media screen and (min-width: 25em){
  html { font-size: calc( 16px + (24 - 16) * (100vw - 400px) / (800 - 400) ); }
}

@media screen and (min-width: 50em){
html { font-size: calc( 16px + (24 - 16) * (100vw - 400px) / (800 - 400) ); }
}

html { font-size: calc(1em + 1vw); }

<h1 style="font-size:10vw;">Responsive Text</h1>

<p style="font-size:5vw;">Resize the browser window to see how the text size scales.</p>