You cannot prove anything about the future... but you can identify trends.
Before Netscape 1.2 came out, it was a common, non-standard hack to use multiple title and body elements to get crude animation. Netscape 1.2 came out, and screwed these pages up. Following standards ensured forwards compatibility with Netscape 1.2.
Before Netscape 2.0 came out, missing quotes on the end of an attribute were detected as errors by Netscape 1.x and compensated for. Netscape 2.0 came out; it did not. Large sections of pages disappeared. Following standards ensured forwards compatibility with Netscape 2.0.
Before Netscape 3.0 came out, people were careless with their ampersands, failing to correctly encode them in URLs, for example. These were detected as errors by the current browsers, and compensated for. Netscape 3.0 came out; it did not. Lots of broken links everywhere. Following standards ensured forwards compatibility with Netscape 3.0.
Before Netscape 4.0 came out, people were still careless with character entities, omitting the trailing semicolon (I believe this was a property of many graphical editors, such as Frontpage). This was detected by the current browsers, and compensated for. Netscape 4.0 came out; it did not. Following standards ensured forwards compatibility with Netscape 4.0.
More recent problems include stylesheets served with an incorrect content-type header, and table-layout images being broken up with lots of little gaps.
This list only includes Netscape behaviour, as that is the only list I have to hand. (Thanks to this article). I'm sure similar things apply to other browsers.
There is plenty of evidence that sticking with standard code results in forwards compatibility.
There are really only two important properties of future browsers:
- They are likely to support at least as much of the specifications as the current version
- Nobody can test in them
Thus, my overwhelming desire is to simply treat future browsers as I would any other browser I couldn't test in: code to standards, and when I get a chance to test, fix up what is necessary.