I spent ages today trying to get quarto to produce a PDF with A4 paper.  The documentation was all very clear, but it didn't work and it took me a long time to realise there was a formatting error in the yaml.  With the error in the yaml text, the whole yaml block was silently ignored, so I didn't know there was an error and hence couldn't understand why it was being ignored.

Example document:

```markdown
---
title: "Test Document"
author: "Me"
format:
  pdf:
    documentclass: article
    papersize: a4
crossref:
  title-delim: :
---

# Title

Text

```

Note that `title-delim` is set to (the default value of) `:`, which without any quotation marks is a yaml error.

If you pass this to quarto, it does this:

```
> quarto render .\test_2.qmd
pandoc
  to: html
  from: markdown
  output-file: test_2.html
  standalone: true
  section-divs: true
  html-math-method: mathjax
  filters:
    - crossref

metadata
  document-css: false
  link-citations: true

Output created: test_2.html
```

Note that the output isn't a PDF and there's no error message to say that the yaml was ignored.
2021-10-20T11:51:40Z
abudden
closed
Feature request: show error message for invalid yaml
https://github.com/quarto-dev/quarto-cli/issues/142