First of all talking about progressive enhancement, what is progressive enhancement? Well progressive enhancement as name suggests it is an enhancement to the progressive browsers, means browsers who understand newer technology's can accept the new code and display the web page in good fashion whereas those browsers who doesn't understand new code still display the web page in fashion which should be accessible to users.
Another question comes is what is web 3.0? Web 3.0 will be an intelligent system which can extract data from web pages automatically without the need of humans. Now how this data can be useful well just for example you search for "pizza" in your browser then your browser will return different pizzas from different local stores and will display its prize and all other information available. And it will automatically rank higher those stores who have good reviews in the past by users.
Now currently technologies we are using like html, XHTML, CSS are not adequate for this purpose and to provide really data in such a way to search engines new technology is required, some of them are currently present like xml can be good way to provide information about data in your site but to be precise w3c has created HTML5. From HTML5 information can be fetched as it has custom tags for many purposes. We can use these technologies in current world by the way of progressive enhancement as there are only few browsers who support this modern era of languages.