Prevent scaling when focusing input element inside iframe on iPhone

When you click on the input field using Safari on iPhone, the browser will zoom in. I could not prevent it.

Every other post on this issue indicates that if the font size of the input element is greater than 16 pixels, then scaling will not occur. This is not true. I set the font size to 29px and confirmed that input and input: focus get the same css application, but the browser is increasing focus. Perhaps this is because there is an interactive need for the iframe.