The title in the code shown is surely just the additional information shown when the user hovers over the link in their browser, and I didn’t think that could be styled with additional tags.
If it can be, surely it’s just a case of adding the open and close H1 tags just inside the quotes?
AFAIK, you can’t… the title attribute is just some additional information, and it’s up to the browser what to do with it (e.g. it may be affected by your browser/desktop theme etc.). Also, a 1st level heading as a tooltip doesn’t seem particularly semantic to me anyway. ;-) If you want some custom hover content, you might do something like