Add README.md and LICENSE.txt

Next step is to update vitest-dev/vitest#4686
with this info, since I just got beaten to opening an issue myself.

LICENSE.txt

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+Copyright 2023 Mike Bland <[email protected]> (https://mike-bland.com)
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the “Software”), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+# Vitest >=1.0.0-beta.5 config.base browser test error demo
+
+Using Vitest 1.0.0-beta.5 or later (including v1.0.1) when vite.config.js
+defines [`base`][] produces the following error when running browser tests:
+
+```text
+TypeError: Failed to fetch dynamically imported module: http://localhost:5173/@id/vitest/utils
+This error originated in "demo.test.js" test file. It doesn't mean the error was thrown inside the file itself, but while it was running.
+```
+
+This demo repo helped me determine the root cause:
+
+- [vitejs/vite#5657: fix!: return 404 for resources requests outside the base
+  path](https://github.com/vitejs/vite/pull/5657) made Vite reject URLs outside
+  of `config.base`.
+- This change became part of [Vite
+5.0.0-beta.8](https://github.com/vitejs/vite/commits/v5.0.0-beta.8)
+- Vitest's dependency on Vite was bumped to `^5.0.0-beta.15` as part of
+  [vitest-dev/vitest#4368: chore!: update vite@5 and
+  rollup@4](https://github.com/vitest-dev/vitest/pull/4368)
+- This change became part of [Vitest
+  1.0.0-beta.5](https://github.com/vitest-dev/vitest/commits/v1.0.0-beta.5)
+
+This issue was filed just before I got the chance, so I'll attach my info there:
+
+- [vitest-dev/vitest: unhandled error with browser tests and modified base
+  #4686](https://github.com/vitest-dev/vitest/issues/4686)
+
+## Reproduction steps
+
+These are distilled from the steps recorded in the commit messages of this repo.
+
+### Initial setup for last good combination
+
+Clone all these repos in the same directory:
+
+```sh
+git clone https://github.com/mbland/vitest-utils-browser-import.git
+git clone https://github.com/vitejs/vite.git
+git clone https://github.com/vitest-dev/vitest.git
+```
+
+Restore Vite and Vitest to the last versions that worked together without
+producing this error:
+
+```sh
+cd vite
+git reset --hard v5.0.0-beta.7
+
+# Not needed the first time, but for each rebuild:
+rm -rf node_modules packages/*/node_modules
+
+pnpm i --ignore-scripts --frozen-lockfile
+pnpm build
+
+cd ../vitest
+git reset --hard v1.0.0-beta.4
+
+# Same as above:
+rm -rf node_modules packages/*/node_modules
+
+pnpm i --ignore-scripts --frozen-lockfile
+pnpm build
+
+cd ../vitest-utils-browser-import
+pnpm i --ignore-scripts
+pnpm test
+```
+
+This should allow the Vitest UI to run and for the test (which does nothing but
+return `null`) to pass.
+
+### Setup failure with Vite commit
+
+To reproduce the error, follow the same steps as above for the first Vite build:
+
+```sh
+cd ../vite
+git reset --hard 40fd2d9bf4073420e6c334f48dc3b63558b688ce
+rm -rf node_modules packages/*/node_modules
+pnpm i --ignore-scripts --frozen-lockfile
+pnpm build
+
+cd ../vitest-utils-browser-import
+pnpm i --ignore-scripts
+pnpm test
+```
+
+This should produce the same failure as above.
+
+### Workaround
+
+The workaround is to delete the `config.base` value in `vitest.config.js`. A
+version of this is already encoded here:
+
+```sh
+pnpm test:workaround
+```
+
+[`base`]: https://vitejs.dev/config/shared-options.html#base