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Note that if the HTML validation fails, then the value of the text box will reset to the previously configured value. The value you attempted to save should be easily to copy from the box inside the warning. If you believe that the validation has made a mistake, you can overwrite it by using the “Skip HTML validation for custom information box content on the next save.”-toggle to skip the validation on the next save. If the stored input HTML is invalid then it can lead to unfortunate results such as in the picture below (the cause of the below error is not closing a div):
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