test/viewport/README.md: rework against the third review

Docker is no longer given as the reason the test is outside make
check; the widths tested as touch devices are named. The full pass
fixed three sentences narrower than the code: where the breakpoints
come from, overflow: clip, and the gateway's fixed 5 ms answer.

Model: fable-5-1

test/viewport/README.md

@@ -17,18 +17,18 @@ 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.
+The test is not part of `make check`: it takes minutes, and `make test` has to
+finish in under 60 seconds.
 
 ## The widths
 
-The widths come from the page's own CSS. `viewports.js` reads the `min-width`
-and `max-width` conditions out of the `@media` blocks in `web/src/styles.css`.
-It also looks in `web/src/main.js` and `web/index.html` for Tailwind's
-responsive class prefixes, such as `md:`, and takes their breakpoints. Each
-breakpoint is tested at one pixel below it, exactly on it, and one pixel above
-it. Today the CSS has one breakpoint, `max-width: 768px`, and no responsive
-prefix is used.
+The breakpoints are read from the page's own files. `viewports.js` reads the
+`min-width` and `max-width` conditions out of the `@media` blocks in
+`web/src/styles.css`. It also looks in `web/src/main.js` and `web/index.html`
+for Tailwind's responsive class prefixes, such as `md:`, and takes their
+breakpoints. Each breakpoint is tested at one pixel below it, exactly on it, and
+one pixel above it. Today the CSS has one breakpoint, `max-width: 768px`, and no
+responsive prefix is used.
 
 A `max-width: 768px` block matches at 768 pixels. Testing both sides of each
 breakpoint catches a layout that switches one pixel too early or too late. A
@@ -50,6 +50,8 @@ Four more widths are fixed, each for a stated reason:
   touch device
 - 1280 by 800 pixels: a desktop window
 
+Every width but 1280 by 800 is tested as a touch device.
+
 ## The checks
 
 A failed check does not stop the others.
@@ -61,9 +63,9 @@ A failed check does not stop the others.
   failure, the widest content is named.
 - `nothing-past-viewport-edge`: no visible element extends past the window's
   edge.
-- `no-clipped-text`: no element hides its text behind `overflow: hidden`. Text
-  cut with an ellipsis is allowed: that cut is a design choice, not a defect.
-  Tailwind's `truncate` cuts host names and URLs that way.
+- `no-clipped-text`: no element hides its text behind `overflow: hidden` or
+  `overflow: clip`. Text cut with an ellipsis is allowed: that cut is a design
+  choice, not a defect. Tailwind's `truncate` cuts host names and URLs that way.
 - `tap-targets-44px`, on touch widths: each measured control is at least 44 by
   44 CSS pixels. The controls are `#pause-btn`, `#interval-select`,
   `#debug-toggle` and the `.pin-btn` in each WAN host row. The check also
@@ -99,7 +101,8 @@ from a fixed table of delays: 2, 45, 123, 456 and 780 milliseconds. Which delay
 a target gets depends on its URL, and about one target in seven fails outright.
 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.
+`http://192.168.1.1` is answered after 5 milliseconds, so the `Local Gateway`
+row appears.
 
 At each width, 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