Highlight feature "gui=strikethrough" does not display strikethrough text.

To Reproduce

1. Run mvim --clean
2. Create or edit an html file
3. Type <strike>some text</strike> or :highlight htmlStrike
4. Observe "gui=strikethrough" yet the strike mark is not on the text.

Expected behavior
The highlight feature "gui=strikethrough" should show a strikethrough

Screenshots
Screenshot attached.

Environment (please complete the following information):

• Vim version:
  VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Mar 3 2020 12:05:00)
  macOS version
  Included patches: 1-319
  Compiled by travis@Traviss-Mac.local
  Huge version with MacVim GUI.
• OS: macOS 10.15.4
• Terminal: MacVim GUI