Elements aren't displaying correctly using VW - css

Question

I'm trying to position elements in a way so that when the browser width is changed, the webpage will scale everything in proportion, but what happens is that they shift a little. I don't understand why. I can adjust this okay using media queries, but they change drastically in mobile browsers. To illustrate what I'm talking about, I created an example in which I'm trying to keep this black text centered inside this green box. From my example, you'll see that scaling the browser on a desktop will keep the text in the box centered pretty well, but when switching to a mobile browser, the text will go out of the box. What can I do to keep it scaling correctly?
I realize that I can just fill the text div with a green background, but you have to understand that this is just an example of what I'm trying to do. The real webpage is much more sophisticated, so that will not be an option. I need to make sure that these divs scale appropriately. Thank you.
I provided an image to show the problem that I'm getting in my phone browser. It's a bit small, but you can see how the black text dips below the green box.
The example website: http://www.marifysworld.com
CSS:
#viewport {
width: device-width;
zoom: 1.0}
#-ms-viewport {
width: device-width}
body {
margin: 0px;
padding: 0px;
background-color: #fffff}
img {
display: block;
margin: 0px;
padding: 0px}
.text {
font-size: 2.25vw;
color: #000000;
text-align: center;
text-size-adjust: 90%}
.box {
width: 23.75%;
height: auto;
position: absolute;
left: 25%;
top: 40vw}
.divtext {
width: 20%;
height: auto;
position: absolute;
left: 26.75%;
top: 42.5vw}
HTML:
<img class="box" src="http://www.marifysworld.com/images/platform/box.jpg" />
<div class="divtext text">
Why won't this div of text stay in the center of the block in mobile browsers?
</div>

Answer 1

Well, you are using positions for your design but it is confusing and not possible.
Here is an idea to make this design work.
Just try it...
HTML:
<div class="box">
<div class="divtext text">
Why won't this div of text stay in the center of the block in mobile browsers?
</div>
</div>
CSS:
#viewport {
width: device-width;
zoom: 1.0}
#-ms-viewport {
width: device-width}
body {
margin: 0px;
padding: 0px;
background-color: #fffff;
}
.box{
background: url('http://www.marifysworld.com/images/platform/box.jpg');
width: 23.75%;
margin: auto;
margin-top: 20%;
}
.divtext {
width: 90%;
padding: 5% 0;
margin: auto;
}
.text {
font-size: 2.25vw;
color: #000000;
text-align: center;
}

Answer 2

Update: initially I thought the problem might be the (not universally supported) text-size-adjust property, but it seems this is unlikely. I leave those thoughts below just in case they are useful to someone else using that property.
Having been unable to reproduce the problem myself but seeing the useful image now put into the question I think we have to look at the actual font and how it is sized and using space. There are quite a few factors which maybe the browsers are setting different defaults for. Here's a few, there may well be more:
font-family - most obvious but is whichever browser is causing the problem using the same default font as browsers not causing the problem? Try setting a specific font and see what happens
Different fonts will take different widths for different characters. Try a monospace font - that will probably overflow - just to demonstrate the issue
kerning - no I don't fully understand how different fonts use it and what they mean by 'normal' (which is probably the browser's default) but that will also alter the space used as will...
..line height - perhaps that needs to be specifically set
font-weight will alter the space used - do all browsers/systems interpret say 400 exactly the same way
I guess there's loads more that may differ between browsers - for example how exactly do they calculate the spacing needed to center text, will they always break a line at the same place etc.
Sorry this is a waffle, but there are so many factors that could make the text overflow and I don't understand them all in enough depth.
Basically what you need is to be able to scale the text div to force it to fit - for that you would need a bit of JS I think (?or is there an equivalent of contain for divs?)
ORIGINAL STUFF:
I am seeing text stay within the green box on a mobile device (IOS Safari) so I imagine the problem you are having is with another mobile device/browser such as Android.
If this is case the area to look at is the use of the CSS property
text-size-adjust: 90%
There are a couple of things to note here:
According to MDN
This is an experimental technology. Check ... carefully before using in production.
This property is intended to be used in the case where a website has not been designed with smaller devices/viewports in mind.
According to MDN, while Chrome on Android implements text-size-adjust fully, Firefox on Android and Safari on IOS do not support the percentage version.
I may be missing something but the question explicitly states that 'the webpage will scale everything in proportion'. Apart from possible inbuilt browser margin and padding on the div, everything is expressed as vw or % so I cannot see anything else that would have an adverse affect on the text positioning.
I also cannot see why this property is being used. It may or may not be causing the problem, but it certainly may affect how text is displayed on some browsers and it seems to be, at best, redundant for a site that is designed with proportionality in mind from the start.