title: html of rmarkdown file (.Rmd) is not displayed correctly in browser
body: <p>When I knit my .Rmd to html, it does not get displayed properly, i. e. it seems like something is wrong with the format (see screenshot below): The two hyperlinks do not work, the font is not sans-serif, there is no table of contents, etc. I used the same code on a different machine 6 months ago and it rendered as expected. However, today it does not. Does anybody have an idea why the html looks ugly? It seems not to depend on the browser I open the html with (tested it in IE and Chrome).</p>
  
  <p><a href="https://i.stack.imgur.com/nSTdS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/nSTdS.png" alt="html output of my .Rmd file"></a></p>
  
  <p>This is the code i use: </p>
  
  <pre><code>---
  title: "my title"
  author: "subtitle"
  date: "my name, `r format(Sys.time(), '%d. %B, %Y')`"
  output:
    html_document:
      code_folding: hide
      highlight: haddock
      number_sections: yes
      toc: yes
      toc_float: yes
  ---
  
  # section
  Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod 
  tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At 
  vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd 
  gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum 
  dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor 
  invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero 
  eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no 
  sea takimata sanctus est Lorem ipsum dolor sit amet.
  
  ```{r, warning = FALSE}
  head(mtcars)
  ```
  </code></pre>
  
  <p>And my session info:</p>
  
  <pre><code>&gt; sessionInfo()
  R version 3.4.2 (2017-09-28)
  Platform: x86_64-w64-mingw32/x64 (64-bit)
  Running under: Windows &gt;= 8 x64 (build 9200)
  
  Matrix products: default
  
  locale:
  [1] LC_COLLATE=German_Switzerland.1252  LC_CTYPE=German_Switzerland.1252    
  LC_MONETARY=German_Switzerland.1252
  [4] LC_NUMERIC=C                        LC_TIME=German_Switzerland.1252    
  
  attached base packages:
  [1] stats     graphics  grDevices utils     datasets  methods   base     
  
  loaded via a namespace (and not attached):
   [1] compiler_3.4.2  backports_1.1.1 magrittr_1.5    rprojroot_1.2   
  htmltools_0.3.6 tools_3.4.2     yaml_2.1.15    
   [8] Rcpp_0.12.14    stringi_1.1.6   rmarkdown_1.8   knitr_1.17      
  stringr_1.2.0   digest_0.6.12   evaluate_0.10.1
  </code></pre>
  
  <hr>
  
  <p><strong>update</strong>: Opened the html in Chrome, pressed F12, and under console it says</p>
  
  <p><a href="https://i.stack.imgur.com/OqsJK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OqsJK.png" alt="Console tab in Chrome"></a></p>