"Internet Explorer 8 and later. When the BUTTON element is submitted in a form, the value depends on the current document compatibility mode. In IE8 mode, the value attribute is submitted. In other document modes and earlier versions of Internet Explorer, the innerText value is submitted."
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@Staszek:
http://msdn.microsoft.com/en-us/library/ms535211(VS.85).aspx says:
"Internet Explorer 8 and later. When the BUTTON element is submitted in a form, the value depends on the current document compatibility mode. In IE8 mode, the value attribute is submitted. In other document modes and earlier versions of Internet Explorer, the innerText value is submitted."