This code gives the following error message: Warning 1 Validation (HTML5): The values permitted for this attribute do not include '1'. Border is a valid attribute of the image tag and 1 is also valid value.
The entire code block is reproduced below with the affected line is
<img src="/Catalog/Images/Thumbs/<%#:Item.ImagePath%>" width="100" height="75" border="1"/></a
<%@ Page Title="Products" Language="C#" MasterPageFile="~/Site.Master" AutoEventWireup="true"
CodeBehind="ProductList.aspx.cs" Inherits="WingTipToys.ProductList" %>
<asp:Content ID="Content1" Conten ...
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