[css-line-grid] Examples do not show headings that snap to the baseline grid

[astearns]

Some feedback: The spec examples seem to show the heading text not adhering to the baseline grid, and it looks like CSS split-leading is at play. This is expected behavior for many people who have strived for vertical rhythm in CSS in the past, but I wouldn’t call it a baseline grid.

The examples in the introduction currently all show cases where the headings do not snap to the baseline grid. There should be at least one example where a heading does snap all of its lines, and perhaps one with box-snap:last-baseline