Fluid input-append in Bootstrap Responsive

Since your input is a .span10, the button should be a .span2.

<div class="input-append">
    <input class="span10" id="msisdn" size="" type="text" name="msisdn"><button class="btn btn-primary span2" type="submit"><i class="icon-plus icon-white"></i></button>
</div>

But the behavior of the button radically changes. You can fix it with this css :

.input-append .btn {
    float: none!important;
    margin-left: 0!important;
}

And to be even more precise, the button should be as large as a .span2 plus the fluidGridGutterWidth. Which is width: 17.02127659% according to the default values.

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