nextssg: cross-entry homepage CSS impact tables (inlineCss, gzip, noai spillover)

nextssg-homepage-cross-entry-css.md

nextssg homepage cross-entry CSS impact (engineer reference)

Build setup: experimental.inlineCss: true, static export compare (apps/benchmark/compare-static-export.mjs).
Baselines: origin/main @ 33f9144; mobile promo branch @ adfd823; thin-ATF experiment (no DS on mobile sync/lazy tagline edges) @ local thin-atf-exp.

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Compressed HTML (gzip) — all three entry points

Entry HTML	main (33f9144)	Branch + DS (adfd823)	Δ vs main	Thin-ATF (local)	Δ vs main	Δ thin vs branch+DS
noai en-US.html	45.9 KiB	70.7 KiB	+24.8 KiB (+54%)	56.3 KiB	+10.3 KiB (+22%)	−14.4 KiB
mobile en-US/home/mobile.html	46.9 KiB	79.1 KiB	+32.2 KiB (+69%)	64.3 KiB	+17.4 KiB (+37%)	−14.8 KiB
desktop en-US/home/desktop.html	32.0 KiB	42.1 KiB	+10.1 KiB (+32%)	42.1 KiB	+10.1 KiB (+32%)	—

Brotli (same builds): noai 24.3 → 35.5 (+11.1 KiB) branch+DS; 24.3 → 29.6 (+5.3 KiB) thin-ATF.

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Uncompressed en-US.html (noai) — inline <style> payload

Metric	main	Branch + DS	Thin-ATF
Raw HTML	~192 KiB	~372 KiB	~249 KiB
Inline <style> total	~40 KiB	~118 KiB	~56 KiB
Δ inline vs main	—	+78 KiB	+16 KiB

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Attribution (noai gzip vs main)

Contributor	Approx. Δ gzip (noai)	Notes
Mobile promo + DS on sync + lazy merge edges	~+25 KiB	Full branch+DS build; customizer/feedback/BTF/DS link-button in noai <style>
Recovered by thin-ATF (no DS on mobile sync; DS only in async islands)	~−14 KiB	70.7 → 56.3 KiB gzip on noai
Residual mobile SCSS spill (no DS on noai)	~+10 KiB	mobile-atb-banner-module still in noai <style>; no customizer/feedback/BTF/DS link-button
Shared layout / graph tax (desktop, no mobile UI)	~+10 KiB	Desktop HTML +10.1 KiB gzip with almost no desktop-specific promo UI

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/home/mobile initial inline CSS (H6 acceptance)

Build	~Inline <style> in home/mobile.html	homepage-customize-feedback-module in initial <style>?	mobile-home-customizer-module in initial <style>?
Branch + DS	~124 KiB	Yes	Yes
Thin-ATF	~56 KiB	No	No

dynamic(ssr: false) defers JS/DOM; CSS stays in first HTML until sync paths stop importing design-system (tagline LinkButton was the demonstrated merge edge).

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Selector presence in noai en-US.html <style>

CSS module / DS	main	Branch + DS	Thin-ATF
mobile-atb-banner-module	—	✓	✓
mobile-home-customizer-module	—	✓	—
homepage-customize-feedback-module	—	✓	—
mobile-below-the-fold-module	—	✓	—
DS link-button	—	✓	—

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Operating rules (short)

1. CRP per route — sync imports only; no @staticpages/design-system on /home/mobile sync graph.
2. DS allowed only under true async islands (customizer → feedback, SAD modal, BTF) with no sync DS bridge.
3. Ship gate — post-build grep + gzip on all three HTML files, not just the route you edited.

rg 'homepage-customize-feedback-module|mobile-home-customizer-module' \
  apps/nextssg/out/en-US.html apps/nextssg/out/en-US/home/mobile.html
rg 'mobile-atb-banner-module' apps/nextssg/out/en-US.html

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One-line summary

Shared inlineCss merge means mobile ATF work cost +25 KiB gzip on noai at peak; thin-ATF left +10 KiB of mobile SCSS spill while ~15 KiB was DS + lazy CSS folded into every entry’s HTML.