Start on web guide (still need to show results)

Author: effigies

docs/_static/chromium_permissions_dark.png

@@ -1 +1,50 @@
 # Using the web validator
+
+The web validator is the primary interface to the validator,
+hosted at <https://bids-standard.github.io/bids-validator/>
+
+![The web interface to the BIDS Validator with the "Select Dataset Files" button highlighted.
+(Dark theme)](../_static/web_entrypoint_dark.png){.only-dark .no-scaled-link width="50%" align=center}
+![The web interface to the BIDS Validator with the "Select Dataset Files" button highlighted.
+(Light theme)](../_static/web_entrypoint_light.png){.only-light .no-scaled-link width="50%" align=center}
+
+To validate a dataset, click the "Select Dataset Files" and select a directory to validate.
+The directory must contain a `dataset_description.json` to be identified as a BIDS dataset.
+
+When you select a directory, you will get a permission prompt,
+verifying that you want to provide access to the directory.
+
+::::{tab-set}
+
+:::{tab-item} Firefox permission prompt
+
+![The Firefox permissions prompt (Dark theme)](../_static/firefox_permissions_dark.png){.only-dark width="50%" align=center}
+![The Firefox permissions prompt (Light theme)](../_static/firefox_permissions_light.png){.only-light width="50%" align=center}
+
+:::
+:::{tab-item} Chrome permission prompt
+:sync: install
+
+![The Chromium permissions notice (Dark theme)](../_static/chromium_permissions_dark.png){.only-dark width="50%" align=center}
+![The Chromium permissions notice (Light theme)](../_static/chromium_permissions_light.png){.only-light width="50%" align=center}
+
+:::
+::::
+
+At present (November 2024), Firefox browsers present the request to "Upload" data,
+while Chromium-based browsers (Google Chrome, Microsoft Edge, and others) request permission
+to "view files".
+In both cases, confirming allows the Javascript application to read the files and perform validation,
+but those files are never sent to a remote server.
+
+## The Legacy Validator
+
+The current BIDS Validator is a complete rewrite of the original BIDS Validator,
+based on the [BIDS Schema].
+The original validator, now called the "legacy validator", can be found at
+<https://bids-standard.github.io/legacy-validator/>.
+
+It is not recommended to use this validator, as it will become increasingly out-of-date,
+but it may be useful for comparative purposes.
+
+[BIDS Schema]: https://bidsschematools.readthedocs.io