Add note about proper usage of tabindex with skip links

[thierryk]

The warning in section 5.4.3 by @patrickhlauke is a great addition but I think the following sentence needs to be seriously emphasized:

However, authors should only make elements focusable if they act as interactive controls or widgets.

or may be followed by something like:

Authors must use tabindex="-1" onto elements that need to receive focus but are not meant to be part of the sequential focus navigation (i.e. the target of a skip link).

I think this is important to mention because it seems many people do not make the difference between "being focusable" and "being keyboard accessible"—assuming that to be focusable, an element has to be part of the tab sequence.

These 2 skip link examples demonstrate the issue:

• knowbility.org where the main section is not part of the sequential focus navigation
• simplyaccessible.com where headings are part of the sequential focus navigation

[patrickhlauke]

I was going to say that this may be unnecessary, as it's mentioned explicitly later on in the same section...but then realised that the note about tabindex="0" that was originally in that warning boxout is also just reiterating/reinforcing what's said later on too. So yes, might as well add a further paragraph to the warning as well.

[patrickhlauke]

I mostly used your excellent initial suggestion there in my proposal, with one major difference: I changed the "must" to "should"

[thierryk]

Yeah—I went back and forth—"must" is a big word.

As a side note, I think the following may also confuse people:

The tabIndex IDL attribute must reflect the value of the tabindex content attribute. Its default value is 0 for elements that are focusable and -1 for elements that are not focusable.

I think in people's mind, this translates to: tabindex="-1" is for elements that are not focusable

[patrickhlauke]

@thierryk good stuff #637