For players who didn't want to download software, 2021 saw two reliable web-based options:
A lightweight, open-source option.
If the schematic is over 100MB, do not use an online viewer. Download or Universal Minecraft Tool instead. minecraft schematic viewer 2021
Note: By 2026, these tools have evolved further. For current recommendations, you would need to check the latest updates. For players who didn't want to download software,