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

The unit-test sentence is split so its subject meets its verb: the
tests call the exported functions; make test runs them with Node's
test runner. The full pass also fixed two claims broader than the code
(every min-width condition; every interactive control), two sentences
whose subject was split from its verb (the ellipsis exception, the
font width), and put back three dropped reasons in one sentence each:
the ellipsis cut is a design choice, the narrow layout must not skip
the checks or the gateway search, and neither test replaces the other.

Model: fable-5-1

test/viewport/README.md

@@ -22,12 +22,13 @@ The test is not part of `make check`: it needs Docker and takes minutes, and
 
 ## The widths
 
-The widths come from the page's own CSS. `viewports.js` reads every `min-width`
-and `max-width` condition 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 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.
 
 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
@@ -61,10 +62,10 @@ A failed check does not stop the others.
 - `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, as Tailwind's `truncate` does on host names and URLs, is
-  allowed.
-- `tap-targets-44px`, on touch widths: every interactive control is at least 44
-  by 44 CSS pixels. The controls are `#pause-btn`, `#interval-select`,
+  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
   demands that each selector matches enough visible elements: one of each single
   control, and one pin button per WAN host row. A renamed class would otherwise
@@ -77,9 +78,10 @@ A failed check does not stop the others.
   is `row` and the graph sits to the right of the details. A row that only
   shrank its fixed 420-pixel column fails.
 - `probing-still-runs`: the page sent at least one check to a target at this
-  width.
+  width. The narrow layout must not skip the checks.
 - `gateway-detection-still-runs`: the `Local Gateway` row is present at this
-  width, so the gateway was looked for and found.
+  width, so the gateway was looked for and found. The narrow layout must not
+  skip that search.
 
 ### The tap-target size
 
@@ -113,8 +115,8 @@ this reaches a filled page without waiting out two rounds at 3 seconds.
 - Physical pixels. The device pixel ratio is set, but antialiasing, colour
   rendering and legibility at a given physical size are not measured.
 - Fonts. The container has DejaVu, not a phone's own monospace font. Text is
-  close to, but not the same width as, text on a real device. A layout that fits
-  here by a few pixels might not fit there.
+  about as wide as on a real device, but not exactly. A layout that fits here by
+  a few pixels might not fit there.
 - Performance. The cost of redrawing the graphs, battery use and behaviour on a
   slow connection are not measured.
 - The browser's own frame. The address bar, safe-area insets and notch cutouts
@@ -133,8 +135,9 @@ this reaches a filled page without waiting out two rounds at 3 seconds.
 
 ## Relation to the unit tests
 
-The unit tests in `test/unit/`, which `make test` runs with Node's built-in test
-runner, call the functions `web/src/main.js` exports, with no browser. This test
-runs the built page in a real browser engine and is the only test here that can
-see a media query. Checks on computed styles and element geometry belong here;
-checks on functions belong in `test/unit/`.
+The unit tests in `test/unit/` call the functions that `web/src/main.js`
+exports, with no browser. `make test` runs them with Node's built-in test
+runner. This test runs the built page in a real browser engine and is the only
+test here that can see a media query. Neither replaces the other. Checks on
+computed styles and element geometry belong here; checks on functions belong in
+`test/unit/`.