View Source line numbering needs selectable nodes within the huge pre element that HTML5 says to generate when loading text/plain. Since the DOM for normal text/plain viewing is part of the interoperable platform but we need something different for product-specific View Source DOM when viewing the source of plain text, implement a separate code path for that. As a first step, the plain text View Source mode should generate a span with a line number id next to each line break within the huge pre.