RMarkdown: Color largest percentage in each row of Kable?

You are trying to take the maximum of character vectors (i.e. c("12.7%", "35.6%", "6.9%")) and in R,

max(c("12.7%", "35.6%", "6.9%"))
#> [1] "6.9%"

and from ?max and ?comparison,

Character versions are sorted lexicographically, and this depends on the collating sequence of the locale in use: the help for ‘Comparison’ gives details.

Character strings can be compared with different marked encodings (see Encoding): they are translated to UTF-8 before comparison.

sort(c("12.7%", "35.6%", "6.9%"), decreasing = TRUE)
#> [1] "6.9%"  "35.6%" "12.7%"

So, we need to convert them to numbers before comparing using readr::parse_number() and to print the cell values with percent format we can use formattable::percent() function.

---
title: "Color Max Percentage"
output:
  html_document: default
  pdf_document: default
---

```{r setup, include = F}
library(tidyverse)
library(knitr)
library(kableExtra)
options(knitr.table.format = "html")

df = data.frame(
  x = c(1, 2, 3, 4, 5),
  a = c("12.7%", "14.0%", "49.2%", "20.4%", "23.2%"),
  b = c("35.6%", "19.0%", "9.1%", "25.5%", "11.2%"),
  c = c("6.9%", "54.1%", "31.3%", "15.4%", "17.5%")
)

df <- df %>%
  # adorn_totals('row') %>%
  mutate(across(a:c, ~ readr::parse_number(.x) / 100)) %>%
  rowwise() %>%
  mutate(across(
    a:c,
    ~ cell_spec(
      formattable::percent(.x, digits = 1),
      format = "html",
      color = ifelse(.x == max(c_across(a:c)), "red", "blue")
    )
  ))

df %>%
  kable(escape = F) %>%
  kable_styling()

```