Fills a shell's holes by string replacement, not HTMLRewriter: a stream cannot report unfilled
holes before the body is on the wire, and an unfilled hole means shell and fragments disagree.
An unfilled placeholder is left in place, never removed: removing it would silently drop a
region, while the empty span can still be filled by BigPipe's own JavaScript.
Fills a shell's holes by string replacement, not HTMLRewriter: a stream cannot report unfilled holes before the body is on the wire, and an unfilled hole means shell and fragments disagree.
An unfilled placeholder is left in place, never removed: removing it would silently drop a region, while the empty span can still be filled by BigPipe's own JavaScript.