Support simpler table structure

[ffoodd]

Fixes #34184

Not quite sure about this, but requiring a tbody doesn't feel right either…

[mdo]

Starting to feel like it might be easier to just target the td and th, then provide some sort of undo for nested tables like .table-reset.

td,
th {
   padding: $table-cell-padding-y $table-cell-padding-x;
   background-color: var(--#{$variable-prefix}table-bg);
   border-bottom-width: $table-border-width;
   box-shadow: inset 0 0 0 9999px var(--#{$variable-prefix}table-accent-bg);
 }

.table-reset {
  td,
  th {
    padding: unset;
    background-color: unset;
    border-bottom-width: unset;
    box-shadow: unset;
  }
}

Also makes me think we can update some of these values to be more CSS variables, and then reset them that way. So instead of the above reset, something like this:

.table-reset {
  --bs-table-bg: unset;
  --bs-table-border: unset;
  // etc
}

[ffoodd]

Indeed. However I'm still concerned this is a valid issue or not.

[ffoodd]

From the discussion in the linked issue, we definetely need to adress this.

Using a reset based on a class would ne some kind of regression since styles could leak again to nested tables.

We coule do the same but target .table .table instead, being closer to existing feature. Any opinion ?

[mdo]

At this point, I'm almost wondering if we go the route of our old selectors and explicitly include td and th to drop the universal selectors?

[ffoodd]

It'd be much simpler indeed, I guess.

Regarding nested tables and leaking styles, the important parts are direct child combinators.