Set overflow-y: hidden for the container and make iframe full width and height using vw and vh css units respectively.
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<html>
<head>
<style>
#ifr {
width: 100vw;
height: 100vh;
}
body {
margin: 0;
padding: 0;
overflow-y: hidden;
}
</style>
</head>
<body>
<iframe
id="ifr"
src="https://exapmle.com"
frameborder="0"
></iframe>
</body>
</html>
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<iframe src="mypage.html" style="border: 0; width: 100%; height: 100%">Your browser doesn't support iFrames.</iframe>
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<iframe src="mypage.html" style="position:fixed; top:0px; left:0px; bottom:0px; right:0px; width:100%; height:100%; border:none; margin:0; padding:0; overflow:hidden; z-index:999999;"> Your browser doesn't support iframes </iframe>