The heading below declares nothing of its own: h1–h6 { break-after: avoid } is the user-agent print default. Its section body asks not to be broken and starts a line above the page boundary, with 410pt of filler above it.
Sentence 1 of the section body, long enough to occupy a line of its own. Sentence 2 of the section body, long enough to occupy a line of its own. Sentence 3 of the section body, long enough to occupy a line of its own. Sentence 4 of the section body, long enough to occupy a line of its own. Sentence 5 of the section body, long enough to occupy a line of its own.
The running text below breaks across the page boundary, so the pass that placed the first heading under it started inside that paragraph rather than at a page top. Each heading still arrives with its own section body.
Sentence 1 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 2 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 3 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 4 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 5 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 6 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 7 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 8 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 9 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 10 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 11 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 12 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 13 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 14 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 15 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 16 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 17 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 18 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 19 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 20 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 21 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 22 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 23 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 24 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 25 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 26 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 27 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 28 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 29 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 30 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 31 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 32 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 33 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 34 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 35 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 36 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 37 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 38 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 39 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure. Sentence 40 of the running text that carries on across the page boundary, long enough to occupy a line of its own at this measure.
Sentence 1 of section 1's body, long enough to occupy a line of its own. Sentence 2 of section 1's body, long enough to occupy a line of its own. Sentence 3 of section 1's body, long enough to occupy a line of its own. Sentence 4 of section 1's body, long enough to occupy a line of its own. Sentence 5 of section 1's body, long enough to occupy a line of its own. Sentence 6 of section 1's body, long enough to occupy a line of its own. Sentence 7 of section 1's body, long enough to occupy a line of its own. Sentence 8 of section 1's body, long enough to occupy a line of its own. Sentence 9 of section 1's body, long enough to occupy a line of its own. Sentence 10 of section 1's body, long enough to occupy a line of its own.
Sentence 1 of section 2's body, long enough to occupy a line of its own. Sentence 2 of section 2's body, long enough to occupy a line of its own. Sentence 3 of section 2's body, long enough to occupy a line of its own. Sentence 4 of section 2's body, long enough to occupy a line of its own. Sentence 5 of section 2's body, long enough to occupy a line of its own. Sentence 6 of section 2's body, long enough to occupy a line of its own. Sentence 7 of section 2's body, long enough to occupy a line of its own. Sentence 8 of section 2's body, long enough to occupy a line of its own. Sentence 9 of section 2's body, long enough to occupy a line of its own. Sentence 10 of section 2's body, long enough to occupy a line of its own.