HTML5 Responsive Web design

In later period of times 2008 - 2011 normally all websites are designed with same width for both header and footer, but now situation is changed, why the design trends are changed drastically ? what are new design trends and ideas ?

What special, if we use the HTML5 tags like <arcticle> <section> <aside> <footer> etc, why can't we do the website with support of <div>, <span>, <p>, <ul>, <li> with out using HTML5 tags with support of <Doctype> only in header instead of using other DTDs.

Normally website designers use 950px -970px width for the main wrapper of website with the height of 1200px, what is new design strategy adopted for this subject.

Your suggestions and response are highly appreciated.

People are now visiting web sites (also) using mobile devices like the tablet and smartphones. So your site layout must be flexible enough to fit into very different screen resolutions. One way to obtain flexibility is to use percentages rather than pixels for width, height, left, top. etc.

With HTML5, you can still use <div>, <span>, <p>, <ul>, <li> etc. You don't have to use <arcticle> <section> and the like.

See also this blog.

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People are now visiting web sites (also) using mobile devices like the tablet and smartphones. So your site layout must be flexible enough to fit into very different screen resolutions. One way to obtain flexibility is to use percentages rather than pixels for width, height, left, top. etc.

With HTML5, you can still use <div>, <span>, <p>, <ul>, <li> etc. You don't have to use <arcticle> <section> and the like.

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Yes i Know, But i would like to suggest a different website for mobile and tablet devices, some times, in certain scenarios we cannot completely design a website in percentage fully , it leads to reduce the flexibility for designer for presenting different layers in a single page.