@jasmineyuan2016

You should be able to use something such as:

<iframe src="https://example.com" style="border: 0; width: 100%; height: 100%"></iframe>

You could options stick this in a CSS file and simply reference a class.

.fullframe {
    border: 0;
    height: 100%;
    width: 100%;
}

<iframe src="https://example.com" class="fullframe"></iframe>

Of course, you’ll most likely still have a gap on each side of your browser window unless you’ve already set a default margin of 0 for body:

body {
    margin: 0;
}