Web page layouts and styles can (and inevitably will) change, but as long as your phrase text still exists on the page, reff.it should be able to find it.
If the content of your reff no longer exists in the page, reff.it will display a warning that it cannot find the phrase.
However, it will still display the URL of the page and the original text you created the reff from.
For the time being, reff.it struggles finding phrases that are nested with HTML (these include phrases that are laden with formatting).
The algorithm driving reff.it is continually being worked upon and improved, so before long it should be able to cope with more complex phrases. For the time being, try to copy and paste phrases that have not been stylised too much.
This is related to the previous question. Whereas in the example above, reff.it completely fails to find the phrase, most of the time reff.it will at least be able to come up with a compromise. If reff.it can't find your entire phrase (due to nested HTML and other problems) it will try again and again, each time knocking off a word from the phrase to make the search broader.
Once again, our algorithm is constantly being refined, so this shouldn't be an issue forever. And when we improve the algorithm, all old reffs are automatically updated too - so, one day, your entire reff should be highlighted with no problems.
This is because the web page preview is not actually the live web page you cited. reff.it instead makes a local copy, so that it can do things like highlight phrases without running into cross-domain issues. This local copy looks fine for some pages, but complex pages might look a bit odd. Sorry about that.