title: Use additional Latex packages for math expressions in RMarkdown `output = "html_document"`
body: <p>I`m aware how to use additional Latex-packages for pdf-format output from .Rmd files using </p>
  
  <pre><code>---
  header-includes:
     - \usepackage{mathtools}
  ---
  </code></pre>
  
  <p>in the YAML header.</p>
  
  <p>However, this does (of course) not work if one specifies <code>output: html_document</code>.</p>
  
  <pre><code>---
  output: html_document
  header-includes:
     - \usepackage{mathtools}
  ---
  </code></pre>
  
  <p>Using additional Latex-Packages could be of interest for <code>output: html_document</code>, too - especially in math expressions (MWE below)</p>
  
  <pre><code>---
  title: "MWE"
  output: html_document
  header-includes:
     - \usepackage{mathtools}
  ---
  
  ## Use "Defined by" Symbol
  
  $$sin(x) \coloneqq \frac{opposite}{hypothenuse}$$
  </code></pre>