Sample HTML5 Code:
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Web programming is a blast!
X-Ray Vision: Seeing the underlying structure of a simple web page
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If we want to see the structure of a web page within a web page (a bit of irony there) we need to use a trick to keep the browser from 'seeing' the code elements, This is done with the little-known 'xmp' tag.
Here is what the code above looks like after it's been rendered by the browser
Possibly more detail here