The Vertical Space value must be an integer greater than zero.
The vspace property of an HTML element is set to a value that is not a positive integer (or number) greater than zero.
To respond to this message
Open this HTML page for editing in the HTML Designer.
Select the element on the Design view surface.
The properties of the element are displayed in the Properties window.
Select the vspace property in the Properties window, and then enter a positive integer greater than zero.
The HTML markup of the selected element is modified. Save these changes to your document.
See Also
Reference
vspace Attribute, vspace Property
Other Resources
Walkthrough: Basic HTML Editing in Visual Studio for Web Forms Pages