Google's notes on mobile-friendly websites

Flash content

Most mobile browsers do not render Flash content. As a result, visitors using mobile devices will not be able to use a page that relies on Flash – they will not see its content or animations, nor be able to use its navigation.

Viewport not configured

Because visitors to your site use a variety of devices with different screen sizes – from large desktop monitors to tablets and small smartphones – your pages should specify a viewport using the meta viewport tag. This tag tells browsers how to adjust the page's dimensions and scaling to suit the device.

Fixed-width viewport

This report shows pages with a fixed-width viewport. Some developers define the viewport with a fixed number of pixels in order to adapt a non-responsive page to the size of common mobile devices.

Content not sized to viewport

This report shows pages that require horizontal scrolling to view their text and images. This happens when pages use absolute values in CSS declarations or use images designed for specific browser widths (e.g. 980px).

Small font size

This report shows pages where the font is too small to be legible, requiring users to zoom in to read the content. After specifying a viewport for your pages, set the font sizes so that text scales properly within it.

Touch elements too close

This report shows the URLs of pages where touch elements, such as buttons and navigation links, are so close together that a mobile user cannot easily tap the desired element without also tapping a neighbouring one.

source: Google