Why any html button cause postback in aspx page?

Check this link, http://www.w3schools.com/tags/tag_button.asp There is a note on page, which says, that different browsers can use different default type for button, if you don't specify it by yourself. Seems like your browser uses "submit".

I know this is an old post. @Dmytro 's post leads in the right direction but is not the exact answer. To prevent the postback you need to make use of the type attribute of the button tag. i.e.

<button type="button">Click Me!</button>