JSFiddle - React, Tailwind, and code Playground

HTML

<iframe src="title.htm" width="100%" height=90 
    style=" 
            border:1px solid grey;" 
    name="title_frame">
    <p>Your browser does not support iframes</p>
</iframe>
<iframe src="hmenu.htm" width="100%" height=70 
    style=" 
            border:1px solid grey;
            margin:0px 0px 0px 0px;" 
    name="hmenu_frame">
</iframe>
<iframe src="startpage.htm" width="100%" height="100%"
    style=" 
            vertical-align:bottom;
            border:1px solid grey;"
    name="content_frame">
</iframe>

CSS

html, body {  
  height: 100%;
  padding: 0;
  margin: 0;
}

body {
  display: flex;
  flex-direction: column;
}
iframe {
  box-sizing: border-box;
}