diff --git a/R/kable_styling.R b/R/kable_styling.R
index 97073e5..b1666f5 100644
--- a/R/kable_styling.R
+++ b/R/kable_styling.R
@@ -257,8 +257,9 @@ htmlTable_styling <- function(kable_input,
     kable_xml_style <- xml_attr(kable_xml, "style")
   }
   if (!is.null(font_size)) {
+    if (is.numeric(font_size)) font_size <- paste0(font_size, "px")
     kable_xml_style <- c(kable_xml_style,
-                         paste0("font-size: ", font_size, "px;"))
+                         paste0("font-size: ", font_size, ";"))
     kable_caption_node <- xml_tpart(kable_xml, "caption")
     if (!is.null(kable_caption_node)) {
       xml_attr(kable_caption_node, "style") <- "font-size: initial !important;"
diff --git a/R/row_spec.R b/R/row_spec.R
index be2fa98..cc1bd5e 100644
--- a/R/row_spec.R
+++ b/R/row_spec.R
@@ -166,8 +166,9 @@ xml_cell_style <- function(x, bold, italic, monospace,
                                    "text-align: ", align, ";")
   }
   if (!is.null(font_size)) {
+    if (is.numeric(font_size)) font_size <- paste0(font_size, "px")
     xml_attr(x, "style") <- paste0(xml_attr(x, "style"),
-                                   "font-size: ", font_size, "px;")
+                                   "font-size: ", font_size, ";")
   }
   if (!is.null(angle)) {
     xml_attr(x, "style") <- paste0(xml_attr(x, "style"),
diff --git a/tests/testthat/_snaps/row_spec.md b/tests/testthat/_snaps/row_spec.md
index 6e1b594..73bd6a0 100644
--- a/tests/testthat/_snaps/row_spec.md
+++ b/tests/testthat/_snaps/row_spec.md
@@ -189,3 +189,37 @@
       \bottomrule
       \end{tabular}
 
+# Issue #576: string font_size in html
+
+    Code
+      cat(data.frame(a = c("foo", "bar", "foo", "bar", "foo", "bar")) %>% kbl() %>%
+        row_spec(3, font_size = "xx-large"))
+    Output
+      <table>
+       <thead>
+        <tr>
+         <th style="text-align:left;"> a </th>
+        </tr>
+       </thead>
+      <tbody>
+        <tr>
+         <td style="text-align:left;"> foo </td>
+        </tr>
+        <tr>
+         <td style="text-align:left;"> bar </td>
+        </tr>
+        <tr>
+         <td style="text-align:left;font-size: xx-large;"> foo </td>
+        </tr>
+        <tr>
+         <td style="text-align:left;"> bar </td>
+        </tr>
+        <tr>
+         <td style="text-align:left;"> foo </td>
+        </tr>
+        <tr>
+         <td style="text-align:left;"> bar </td>
+        </tr>
+      </tbody>
+      </table>
+
diff --git a/tests/testthat/test-row_spec.R b/tests/testthat/test-row_spec.R
index fe4a8ac..025400f 100644
--- a/tests/testthat/test-row_spec.R
+++ b/tests/testthat/test-row_spec.R
@@ -77,4 +77,15 @@ test_that("awesome table example", {
             row_spec(c(1:7, 11:12) - 1, extra_latex_after = "\\rowcolor{gray!6}") %>%
             collapse_rows(1, latex_hline = "none")
     )
+})
+
+
+
+test_that("Issue #576: string font_size in html", {
+    expect_snapshot(
+        data.frame(a = c("foo", "bar", "foo", "bar", "foo", "bar")) %>%
+            kbl() %>%
+            row_spec(3, font_size = "xx-large") |>
+            cat()
+    )
 })
\ No newline at end of file