To make a box responsive, you can use CSS (Cascading Style Sheets) to specify its dimensions in relative units such as percentages or em, instead of fixed units such as pixels. This way, the box will adjust its size based on the screen size of the device it is viewed on. Here's an example of how to make a div box responsive using CSS:

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.box {
  width: 80%; /* set the width of the box to 80% of its parent container */
  padding-bottom: 80%; /* set the height of the box to be the same as its width */
  background-color: blue;
}

In this example, the box will adjust its width and height based on the size of the parent container. The padding-bottom property is used to specify the height of the box, which is set to be the same as the width using the percentage value. Using the same percentage for the width and height will make the box a square. You can adjust these percentage values to fit your specific needs.

Here's a link to a Codepen with the example above: https://codepen.io/shecodes/pen/rNKObeO

I'm assuming you mean changing the dimensions of an element from other units to pixels. To do this, you can use CSS.

Let's say your element has a width of 50% and a margin of 1em. To convert these to pixels, you'll need to know the width of the parent element and the font-size of your document. Once you have those values, you can calculate the width and margin in pixels using the formula:

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value in pixels = (value in other unit) * (parent width in pixels) / 100

For example, let's say the parent element has a width of 800px and the font-size is 16px. To convert the width of the element to pixels, you would do:

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width in pixels = 50% * 800 / 100 = 400px

To convert the margin to pixels, you would need to know the value of 1em in pixels (which is equal to the font-size). Let's say 1em is equal to 16px:

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margin in pixels = 1em * 16px = 16px

So your updated CSS would be:

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element {
  width: 400px;
  margin: 16px;
}

Remember to adjust the values based on your specific case.

iframe is not a part of CSS, it’s actually an HTML element. An iframe is an HTML element used to embed another HTML document within the current one. It allows you to embed content, such as a YouTube video or an interactive map, from another site into your web page. You can use CSS to style the iframe element, such as changing its dimensions or adding a border around it.

Example:

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<iframe src="https://www.youtube.com/embed/dQw4w9WgXcQ"
        title="YouTube video player"
        frameborder="0"
        allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture"
        allowfullscreen></iframe>

In the above code block, you can see how an iframe is used to embed a YouTube video. The src attribute specifies the URL of the video to be embedded. The frameborder attribute sets whether the frame should have a border or not. Finally, the allowfullscreen attribute allows the video to be played in full-screen mode.

To make all images the same size, you can use CSS to set the width and height of the images. You can also use a JavaScript library like jQuery to select all images and set the dimensions. Here's an example using CSS:

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img {
  width: 300px;
  height: 200px;
}

This will set the width of all <img> tags to 300 pixels and the height to 200 pixels. You can adjust these values to fit your specific needs.

Here's a link to more information on using CSS to adjust image size: https://www.w3schools.com/css/css_dimension.asp