Question

For an example

Is <a href="#"><h1>Heading</h1></a> valid in HTML5?

Answer 1

yes what you’ve written is valid in HTML5, but it’s not all inline elements, I think it’s just <a> ‘s it applies to..

Reference: “Block-level” links in HTML5

Tip: if using this set the <a> to display: block; or there may be unintended visual styling results : Source: Test Case

Update:

It is "disallowed" for other "block in inline" combinations where "default styles are likely to lead to confusion" – explanation is here:

Cases where the default styles are likely to lead to confusion

Certain elements have default styles
or behaviors that make certain
combinations likely to lead to
confusion. Where these have equivalent
alternatives without this problem, the
confusing combinations are disallowed.

For example, div elements are rendered
as block boxes, and span elements as
inline boxes. Putting a block box in
an inline box is unnecessarily
confusing; since either nesting just
div elements, or nesting just span
elements, or nesting span elements
inside div elements all serve the same
purpose as nesting a div element in
a span element, but only the latter
involves a block box in an inline box,
the latter combination is disallowed.