Clienside validation before server side

My issues:

1) Postback always occurs

2) Is that the correct way to see if requiredFieldValidator is valid client side?

What I would like:

User clicks the visible click button.

This invokes isValid().

If page is valid then invoke server side function.

If page is invalid do alert.

<asp:Button ID="btnSubmit" runat="server" Text="Submit" ToolTip="Submit" CausesValidation="true" CssClass="blueNew buttonNew" 
                OnClientClick="isValid(); return false;" />

<div style="display: none;">
    <asp:Button ID="hiddenBtnSubmit" runat="server"  OnClick="btnSubmit_Click" />
 </div>

 <script>
function isValid() {
    if (Page_IsValid) {
        var button = document.getElementByID("<%=hiddenBtnSubmit.ClientID%>");
        button.click();
    } else {
        if (!reqFirstName.IsValid) {
            alert("First name is invalid");
        }
    }      
}
 </script>

Nope, you just have to add the validators like this:

<asp:TextBox runat="server" ID="txtEmail" />
<asp:RequiredFieldValidator
    ErrorMessage="The email is required"
    ToolTip="The email is required"
    Text="(*)"
    ControlToValidate="txtEmail"
    runat="server" />
<asp:ValidationSummary runat="server" DisplayMode="BulletList" ShowMessageBox="true" />

And the validation will be executed on the client whenever you try to submit your form, you do not need to manually execute the validation

The output will be something like:

Edit 1

<script type="text/javascript">
    function validate() {
        var val = Page_ClientValidate('');
        if (!val) {
            for (i = 0; i < Page_Validators.length; i++) {
                if (!Page_Validators[i].isvalid) {
                    $("#" + Page_Validators[i].controltovalidate).qtip();
                }
            }
        }

        return val;
    }
</script>

<asp:Button OnClientClick="return validate();" Text="Post" runat="server" />