As an example, when one follows 
http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#processing-model-4
it gives no clues that the algorithm might not apply to some input element types.

In order to know that, one needs to look at the specific type, for example
http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#hidden-state-%28type=hidden%29
and see that 'autocomplete' is listed as  'do not apply'.
Then one needs to find the definition for that 
http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#do-not-apply and from there to actually understand what
the whole thing means, 
http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#common-input-element-attributes

To get better readability, I suggest having a precondition step in each algorithm which applies conditionally.
So, before "1. If the element has no autocomplete attribute, then jump to the step labeled default."
there would be something like
Precondition: abort this algorithm immediately if the following preconditions don't hold:
              'autocomplete' attribute <a href="http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#common-input-element-attributes">applies to</a> the element

Precondition should be used consistently, so that whenever it isn't there, reader would know that there are no preconditions.

I can see how the current spec is easier for the spec author, but it certainly is rather hard for follow for the reader.
And as with code, specs should favor readers.