Why does the flextable looks different in R Viewer vs. in HTML

Question

How can you control line thickness in html outputs using flextable? It looks fine in the RStudio viewer. I think this might not have to do with flextable and might just be a problem with however flextable gets turned to HTML, but not sure where that occurs.

library(dplyr)
library(flextable)
library(officer)

iris %>% head(n=4) %>% flextable() %>% 
  bg(bg = "#00557F", part = "header") %>%
  bold(part = "header") %>%
  color( color = "white", part = "header") %>%
  border_outer(border = big_border, part = "all" ) %>%
  border_inner_h( border = std_border, part = "all" ) %>%
  border_inner_v( border = std_border, part = "all" ) %>% 
  autofit() %>% save_as_html(path = 'ex.html')