io_scene_x3d importer/exporter texture coordinate precision is inadequate for some applications.
The x3d importer/exporter is hardwired to 4 decimal digit precision in generated texture coordinates.
For example, a finely meshed filleted edge on a large object with a single texture could easily have coordinate spacing in (u,v) of less than 1x10^-4, in which case you get a (u,v) space triangle with two vertices that have identical coordinates in the file, and thus zero area, etc. etc. which makes for all sorts of potential problems when the file is read back in.
The relevant lines from https://developer.blender.org/diffusion/BA/browse/master/io_scene_x3d/export_x3d.py
813: fw('%.4f %.4f ' % x3d_v[slot_uv])
931: fw('%.4f %.4f ' % uv[:])
Obviously adjusting the precision to (say) 6 digits is a trivial modification.
Here is a patch that makes the precision default to 6, but also be adjustable: