Cross browser semibold

[gavin-ts]

Summary

Fixes issue of markdown headers being rendered incorrectly outside of chrome.

Details

• fixes The content is not fully displayed when I used this Markdown text template. #1162
• using font-weight: 600 for markdown header elements was using browser's font-synthesis which would render differently in firefox and safari, leading to an overflow
• adds semibold fonts to use in place of font-weight: 600
• fixed text measurement to use semibold instead of bold (this was slightly over-measuring)
• adds md_font_weight regression test
• fonts from:
  • https://github.com/adobe-fonts/source-code-pro/tree/72d3d0eb3567e68669a2b97904a1274ac3b376fa/WOFF/TTF
  • https://github.com/adobe-fonts/source-sans/tree/ce77773581f4d454f0fa985c073bb25c721bfcf5/WOFF/TTF
  • using similar process as render: fonts: add bold + italic styles for Source Code Pro #710
  • Note: source sans 3 is slightly different from source sans pro so we want the versions linked above https://github.com/adobe-fonts/source-sans/tree/release/TTF

using font-synthesis: none results in font rendered at default font-weight 400

before/after chrome

before/after safari

before/after firefox

e2ereport