In bidi default strategy, make steps consistent with each other

spec/formatting.md

@@ -930,31 +930,39 @@ Implementations MAY supply a _bidirectional isolation strategy_ that performs no
 
 The _Default Bidi Strategy_ is defined as follows:
 
+1. Let `out` be the empty string.
 1. Let `msgdir` be the directionality of the whole message,
    one of « `'LTR'`, `'RTL'`, `'unknown'` ».
    These correspond to the message having left-to-right directionality,
    right-to-left directionality, and to the message's directionality not being known.
-1. For each _expression_ `exp` in _pattern_:
-   1. Let `fmt` be the formatted string representation of the _resolved value_ of `exp`.
-   1. Let `dir` be the directionality of `fmt`,
-      one of « `'LTR'`, `'RTL'`, `'unknown'` », with the same meanings as for `msgdir`.
-   1. Let the boolean value `isolate` be
-      True if the `u:dir` _option_ of the _resolved value_ of `exp` has a value other than `'inherit'`,
-      or False otherwise.
-   1. If `dir` is `'LTR'`:
-      1. If `msgdir` is `'LTR'` in the formatted output
-         and `isolate` is False,
-         let `fmt` be itself
-      1. Else, in the formatted output,
-         prefix `fmt` with U+2066 LEFT-TO-RIGHT ISOLATE
-         and postfix it with U+2069 POP DIRECTIONAL ISOLATE.
-   1. Else, if `dir` is `'RTL'`:
-      1. In the formatted output,
-         prefix `fmt` with U+2067 RIGHT-TO-LEFT ISOLATE
-         and postfix it with U+2069 POP DIRECTIONAL ISOLATE.
+1. For each part `part` in _pattern_:
+   1. If `part` is a plain literal (text) part, append `part` to `out`.
+   1. Else if `part` is a _markup_ _placeholder_:
+      1. Let `fmt` be the formatted string representation of the _resolved value_ of `part`.
+         Note that this is normally the empty string.
+      1. Append `fmt` to `out`.
    1. Else:
-      1. In the formatted output,
-         prefix `fmt` with U+2068 FIRST STRONG ISOLATE
-         and postfix it with U+2069 POP DIRECTIONAL ISOLATE.
+      1. Let `fmt` be the formatted string representation of the _resolved value_ of `part`.
+      1. Let `dir` be the directionality of `fmt`,
+         one of « `'LTR'`, `'RTL'`, `'unknown'` », with the same meanings as for `msgdir`.
+      1. Let the boolean value `isolate` be
+         True if the `u:dir` _option_ of the _resolved value_ of `part` has a value other than `'inherit'`,
+          or False otherwise.
+      1. If `dir` is `'LTR'`:
+         1. If `msgdir` is `'LTR'` and `isolate` is False:
+            1. Append `fmt` to `out`.
+         1. Else:
+            1. Append U+2066 LEFT-TO-RIGHT ISOLATE to `out`.
+            1. Append `fmt` to `out`.
+            1. Append U+2069 POP DIRECTIONAL ISOLATE to `out`.
+      1. Else if `dir` is `'RTL'`:
+         1. Append U+2067 RIGHT-TO-LEFT ISOLATE to `out.`
+         1. Append `fmt` to `out`.
+         1. Append U+2069 POP DIRECTIONAL ISOLATE to `out`.
+      1. Else:
+         1. Append U+2068 FIRST STRONG ISOLATE to `out`.
+         1. Append `fmt` to `out`.
+         1. Append U+2069 POP DIRECTIONAL ISOLATE to `out`.
+1. Emit `out` as the formatted output of the message.