Read the exported file, not just the design preview
A design preview and an exported mesh are different checkpoints. Open the actual STL you plan to import. Inspect the front, side and underside, then compare its X, Y and Z extents with the dimensions you intended. The viewer supports binary and single-solid ASCII STL files up to 20 MiB and 250,000 triangles. It reads coordinates locally and leaves the original unchanged.
Try the documented relief sample first: it contains 24,572 triangles and spans approximately 120 × 80.1563 × 5.9843 coordinate units. These are measured file properties. They do not establish that somebody printed or carved this sample successfully. The same inspection works for a file from another tool, so you can compare exports with a consistent set of checks.
Confirm units where you will make the part
STL stores triangle coordinates without a standardized length-unit field. A span of 120 may be interpreted as millimeters or another unit by the receiving software. This viewer therefore reports coordinate extents, not a certified physical size. Check the import unit and final dimensions in your slicer or CAM application before generating machine instructions.
The original coordinates are used for the report. For navigation, the visual mesh is centered in the viewer; that does not modify the file. Drag or use the focused preview's arrow keys to rotate, zoom to inspect details, and reset the view if you lose the model. If WebGL is unavailable, the file statistics remain useful even without the interactive picture.
What a successful preview does not establish
A mesh can render while still containing holes, overlapping faces, self-intersections or walls too thin for the intended process. This viewer reports exactly zero-area triangles as a basic warning. It does not run a full topology audit, measure minimum wall thickness or repair the model. Zero flagged triangles is not a print-ready or watertight certificate.
Continue in a slicer or CAM tool: check layers or toolpaths, stock size, orientation, support needs and process clearances. Then make a small test using the intended material. If your starting point is a photograph rather than an STL, use the image-to-STL workflow separately. A viewer should inspect a file you have; it should not ask you to buy a generation job just to open it.