Question

what is the reason that <img> tag cannot be nested in <a> tag by HTML5 specifications

<a href="#">
    <img src="src" alt="alt"/>
</a>

visual studio 2010 sp1 say that

Answer 1

It is allowed, but not necessarily in every situation.

An a element’s content model is transparent, which means, its content model is the same as its parent’s content model.

Respective a specification:

Content model:
Transparent, but there must be no interactive content descendant.

Respective img specification:

Contexts in which this element can be used:
Where embedded content is expected.

So if the content model of the a element’s parent is phrasing content or embedded content, the img element is allowed:

As a general rule, elements whose content model allows any phrasing content should have either at least one descendant text node that is not inter-element whitespace, or at least one descendant element node that is embedded content.