The mechanism which enables controls to maintain their state that is called View State.
To select or clear it, We need to set the AutoPostBackproperty to True state to have a CheckBoxcontrol submit back to the server.
<asp:CheckBox ID="CheckBox1" runat="server" AutoPostBack="True"/>
The ASP.net run time uses the hidden field called __VIEWSTATE to store the values for the controls. This hidden field is sent with each postback to the server, where it's unpacked and then used to repopulate the controls in the page with their previous values.
The first way open the Solution Explorer and then Right-click your project,and choose Add Existing Item and then browse for the item(s) you want to add.
The Second alternate way, you can drag and drop files from Windows Explorer or from your desktop directly into a VWD project.
Otherwise you could put the files directly in the project's folder using Windows Explorer.
XHTML is a stricter version of HTML that is rewritten with XML rules. In html sometimes we can use the code without any structure but in XHTML we should follow the rules and structure strictly, so that we can get the results that we are expecting. For example in XHTML elements must be properly nested and Attribute names must be in lower case and documents must have one root element and etc.