test/viewport/README.md rewritten sentence by sentence

What the test does comes first, then the widths, the checks, the
canned network, what it cannot check, the files, and its relation to
the unit tests. The repo's history and issue references are gone;
every sentence is under about 30 words.

Model: fable-5-1

test/viewport/README.md

@@ -7,15 +7,15 @@ in a real browser:
 make frontend-viewport-test
 ```
 
-First it runs the unit tests. Then it builds the page into `dist/`. It serves
-`dist/` with the nginx image and the `configs/nginx.conf` that the Docker image
-ships. Before the browser starts, it fetches `/` and `/no-such-page`, which
-nginx answers with the page. Both must carry `Cache-Control: no-cache` and a
-`Content-Security-Policy` header. Then it starts a headless Chrome in a
-container and opens the page in it at each width. For each width it measures the
-layout inside the page and checks the measurements in Node. The checks decide
-pass or fail. Screenshots and a `results.json` land in `tmp/viewport/`, for a
-person to look at when a check fails.
+It runs the unit tests, builds the page into `dist/`, and serves it from the
+same nginx image, with the same `configs/nginx.conf`, as the Docker image.
+Before the browser starts, it checks that `/`, and a path nginx answers with the
+page, are served with `Cache-Control: no-cache` and a `Content-Security-Policy`
+header. Then it starts a headless Chrome in a container and opens the page in it
+at each width. For each width it measures the layout inside the page and checks
+the measurements in Node. The checks decide pass or fail. Screenshots and a
+`results.json` land in `tmp/viewport/`, for a person to look at when a check
+fails.
 
 The test is not part of `make check`: it needs Docker and takes minutes, and
 `make test` has to finish in under 60 seconds.
@@ -51,7 +51,7 @@ Four more widths are fixed, each for a stated reason:
 
 ## The checks
 
-A failed check does not stop the others.
+Every check runs at every width, and none stops the others.
 
 - `app-rendered`: at least 10 WAN host rows exist, and at least 5 rows show a
   number as their latency. Without this, the other checks could pass against a
@@ -99,7 +99,7 @@ So the rows show one-, two- and three-digit latencies and some unreachable rows,
 and the layout is tested against that spread. The request to
 `http://192.168.1.1` is answered, so the `Local Gateway` row appears.
 
-At each width, the test sets the interval to 1 second and waits until at least 5
+On each page, the test sets the interval to 1 second and waits until at least 5
 rows show a latency before it measures. The page discards its first round, so
 this reaches a filled page without waiting out two rounds at 3 seconds.