### Bug description

When tables get rendered without a `colgroup` element (so whenever columns do not get a specific size), the border at the top of the table doesn't show. For example, in a new website with this `index.qmd`:

``` md
---
title: "Test"
---

| a | a | a |
| - | - | - |
| a | a | a |

| aaaaaaaaa  | aaaa   | aaaaaaaaaaa |
| -          | -      | ---         |
| aaaaaaaaaaaaa | aaaaaaaaaaaa | aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |

| aaaaaaaaa  | aaaa   | aaaaaaaaaaa |
| -          | -      | ---         |
| aaaaaaaaaaaaa | aaaaaaaaaaaaaaaa | aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
```

only the third table's top border is rendered. This is because the top border CSS rule is defined as `.table>:not(:first-child)` and the two first tables do not have the `colgroup` element (I think this is Pandoc?).

I'm using Quarto v1.2.242.

### Checklist

- [X] Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
- [X] Please [format your issue](https://quarto.org/bug-reports.html#formatting-make-githubs-markdown-work-for-us) so it is easier for us to read the bug report.
- [X] Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
- [X] Please document the operating system you're running. If on Linux, please provide the specific distribution.
cscheid
2022-10-28T10:49:29Z
tomvaneyck
bug tables
v1.3
closed
Top border table does not get rendered without `colgroup`
https://github.com/quarto-dev/quarto-cli/issues/3084