Inconsistent strikethrough behavior with different nesting orders of markup

[Xuanghdu]

When combining strikethrough (~~) with other markup, the behavior differs depending on the nesting order.

• When strikethrough is nested inside, strikethrough renders correctly:

  • **~~a~~**b renders as **~~a~~**b
  • <b>~~a~~</b>b renders as ~~a~~b

• When strikethrough wraps other markup, no strikethrough is produced:

  • ~~**a**~~b renders as ~~a~~b
  • ~~<b>a</b>~~b renders as ~~a~~b

However, adding a space after makes it work:

• ~~**a**~~ b renders as ~~a~~ b
• ~~<b>a</b>~~ b renders as ~~a~~ b

This inconsistency seems unexpected, as the only difference is whether strikethrough is the outer or inner markup.

Would it be possible to:

1. Confirm if this different behavior based on nesting order is intended
2. If not intended, make the behavior consistent
3. If intended, document this behavior in the GFM spec

[UziTech]

To me it is surprising that **~~a~~**b renders as **~~a~~**b because it doesn't work for other inline elements like **_a_**b which renders like **a**b.

It seems like it should render like **~~a~~**b. The reason the underscores render like that is because of CommonMark spec's flanking rules