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Note that controls that are optional do not cause the submit button to be disabled unless the value entered is invalid. As the figure above shows, there are no values in the optional Email Address and Phone fields but the form may be submitted. However, if the user enters an invalid value in one of the fields (required or not), the submit button will again be disabled by .
And here is a form missing values in required fields. Therefore again the submit button in disabled.
When a form has sections or tabs, the submit button may be disabled due to invalid controls that are not currently visible. For example, a section that is collapsed or a tab that is not currently selected may have an invalid control. This is indicated in the header the section and tab label color changing to red.
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