Assuming, xxxx is always a positive, unique number, has someone done tests on breaking tabindex. For example, is any length of number acceptable in major browsers? Is there a maximum number of elements on a page?
HTML 4.01 sets a maximum value of 32767 for the tabindex attribute. HTML5 LC and WHATWG HTML do not set such a restriction, but this appears to be an oversight, and I have proposed that it be fixed.
Browsers actually enforce the limit, though in incompatible ways. Chrome and Firefox interpret larger values as 32767 or (for very large numbers) 0, IE as negative numbers (except that 32768 is taken as 0).
There is no limit on the number of elements on a page. Neither is there any limit on the number of elements that participate in the tabbing order. You can set the order explicitly with the tabindex attribute “only” for 32767 elements. If you need more elements in the tabbing order, you need specify the rest so that they appear (in source HTML) in that order (and after elements with a positive tabindex value).