[html] <strong> vs. font-weight:bold & <em> vs. font-style:italic

The <em> element - from W3C (HTML5 reference)

YES! There is a clear difference.

The <em> element represents stress emphasis of its contents. The level of emphasis that a particular piece of content has is given by its number of ancestor <em> elements.

<strong>  =  important content
<em>      =  stress emphasis of its contents

The placement of emphasis changes the meaning of the sentence. The element thus forms an integral part of the content. The precise way in which emphasis is used in this way depends on the language.

Note!

  1. The <em> element also isnt intended to convey importance; for that purpose, the <strong> element is more appropriate.

  2. The <em> element isn't a generic "italics" element. Sometimes, text is intended to stand out from the rest of the paragraph, as if it was in a different mood or voice. For this, the i element is more appropriate.

Reference (examples): See W3C Reference