The best solution, since your server is 8.5.3 with an openNTF ExtLib 8.5.3 release, is to make your Designer client match your server version. That is, use Domino Designer 8.5.3, with that exact same openNTF Extlib 8.5.3 release installed into your Domino Designer install.
If you have some reason why you need to use the Domino Designer 9.0 client, then you have the possibility that the applications you develop will contain controls/tags or properties that were added in 9.0 and were not present in 8.5.3. When you try to run such applications, you'll get errors.
The error message when you use a 9.0 XPages runtime feature on an 8.5.3 server:
The generated page version {9.0.0} is not compatible with the server version {8.5.3}, for the page {/home.xsp}.
The error message when you use more recent ExtLib feature on a server containing an older ExtLib version:
The page {/home.xsp} requires {com.ibm.xsp.extlib.library} version {9.0.0.v00_00} or higher. The current library version is {8.5.30006}.
[The { and } symbols don't appear in the actual output, they just indicate the inserted text there.]
So you can actually use Domino Designer 9.0 with an 8.5.3 server, only you'll have to fix such problems whenever they occur.
When using Designer 9.0, you don't need to install ExtLib from openNTF, because the ExtLib library is already present in Designer 9.0. If you are using the ExtLibX features (Relational/JDBC data sources, the Social SBT plugin) they are not included in Designer 9.0, and would need to be installed from openNTF.