Here is a hack I used on one of my projects:

select {
    font-size: 2.6rem; // 1rem = 10px
    ...
    transform-origin: ... ...;
    transform: scale(0.5) ...;
}

Ended up with the initial styles and scale I wanted but no zoom on focus.

By the way, if you use Bootstrap, you can just use this variant:

.form-control {
  font-size: 16px;
}

After a while of while trying I came up with this solution

// set font-size to 16px to prevent zoom 
input.addEventListener("mousedown", function (e) {
  e.target.style.fontSize = "16px";
});

// change font-size back to its initial value so the design will not break
input.addEventListener("focus", function (e) {
  e.target.style.fontSize = "";
});

On "mousedown" it sets font-size of input to 16px. This will prevent the zooming. On focus event it changes font-size back to initial value.

Unlike solutions posted before, this will let you set the font-size of the input to whatever you want.

After reading almost every single line here and testing the various solutions, this is, thanks to all who shared their solutions, what I came up with, tested and working for me on iPhone 7 iOS 10.x :

@media screen and (-webkit-min-device-pixel-ratio:0) {
    input[type="email"]:hover,
    input[type="number"]:hover,
    input[type="search"]:hover,
    input[type="text"]:hover,
    input[type="tel"]:hover,
    input[type="url"]:hover,
    input[type="password"]:hover,
    textarea:hover,
    select:hover{font-size: initial;}
}
@media (min-width: 768px) {
    input[type="email"]:hover,
    input[type="number"]:hover,
    input[type="search"]:hover,
    input[type="text"]:hover,
    input[type="tel"]:hover,
    input[type="url"]:hover,
    input[type="password"]:hover,
    textarea:hover,
    select:hover{font-size: inherit;}
}

It has some cons, though, noticeably a "jump" as result of the quick font size change occuring between the "hover"ed and "focus"ed states - and the redraw impact on performance

This worked for me:

input, textarea {
    font-size: initial;
}

Add user-scalable=0 to viewport meta as following

<meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=0">

Worked for me :)

I recently (today :D) had to integrate this behavior. In order to not impact the original design fields, including combo, I opted to apply the transformation at the focus of the field:

input[type="text"]:focus, input[type="password"]:focus,
textarea:focus, select:focus {
  font-size: 16px;
}

解决此问题的正确方法是将元视口更改为：

<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=0"/>

总之，答案是：将表单元素的字体大小设置为至少16px

如果您的网站是为移动设备设计的，则可以决定不允许扩展。

<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=0" />

这解决了您的移动页面或表单将“浮动”的问题。

@media screen and (-webkit-min-device-pixel-ratio:0) { 
  select:focus,
  textarea:focus,
  input:focus {
    font-size: 16px;
    background: #eee;
  }
}

新增：IOS仍会缩放，除非您在输入中使用16px且没有焦点。

@media screen and (-webkit-min-device-pixel-ratio:0) { 
  select,
  textarea,
  input {
    font-size: 16px;
  }
}

我添加了背景，因为IOS在选择中未添加任何背景。

You can prevent Safari from automatically zooming in on text fields during user input without disabling the user’s ability to pinch zoom. Just add maximum-scale=1 but leave out the user-scale attribute suggested in other answers.

It is a worthwhile option if you have a form in a layer that “floats” around if zoomed, which can cause important UI elements to move off screen.

<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">