diff --git a/content/urldb.c b/content/urldb.c
index 5b5172d6a..6da257aa4 100644
--- a/content/urldb.c
+++ b/content/urldb.c
@@ -1056,7 +1056,7 @@ void urldb_set_thumbnail(const char *url, struct bitmap *bitmap)
  * \param url Absolute URL to search for
  * \return Pointer to thumbnail data, or NULL if not found.
  */
-const struct bitmap *urldb_get_thumbnail(const char *url)
+struct bitmap *urldb_get_thumbnail(const char *url)
 {
 	struct path_data *p;
 
diff --git a/content/urldb.h b/content/urldb.h
index d7e485d3d..08b0b0804 100644
--- a/content/urldb.h
+++ b/content/urldb.h
@@ -93,7 +93,7 @@ bool urldb_get_cert_permissions(const char *url);
 
 /* Thumbnail handling */
 void urldb_set_thumbnail(const char *url, struct bitmap *bitmap);
-const struct bitmap *urldb_get_thumbnail(const char *url);
+struct bitmap *urldb_get_thumbnail(const char *url);
 
 /* URL completion */
 void urldb_iterate_partial(const char *prefix,
diff --git a/desktop/history_core.c b/desktop/history_core.c
index 0550f596d..2081d132d 100644
--- a/desktop/history_core.c
+++ b/desktop/history_core.c
@@ -225,7 +225,7 @@ void history_add(struct history *history, struct content *content,
 	struct history_entry *entry;
 	char *url;
 	char *title;
-	const struct bitmap *bitmap;
+	struct bitmap *bitmap;
 
 	assert(history);
 	assert(content);
diff --git a/desktop/knockout.c b/desktop/knockout.c
index 842c4cb85..f57ae5b54 100644
--- a/desktop/knockout.c
+++ b/desktop/knockout.c
@@ -55,7 +55,7 @@ static bool knockout_plot_path(float *p, unsigned int n, colour fill,
 static bool knockout_plot_fill(int x0, int y0, int x1, int y1, colour c);
 static bool knockout_plot_clip(int clip_x0, int clip_y0,
 		int clip_x1, int clip_y1);
-static bool knockout_plot_text(int x, int y, struct css_style *style,
+static bool knockout_plot_text(int x, int y, const struct css_style *style,
 		const char *text, size_t length, colour bg, colour c);
 static bool knockout_plot_disc(int x, int y, int radius, colour colour, bool filled);
 static bool knockout_plot_arc(int x, int y, int radius, int angle1, int angle2,
@@ -166,7 +166,7 @@ struct knockout_entry {
 		struct {
 			int x;
 			int y;
-			struct css_style *style;
+			const struct css_style *style;
 			const char *text;
 			size_t length;
 			colour bg;
@@ -757,7 +757,7 @@ bool knockout_plot_clip(int clip_x0, int clip_y0,
 }
 
 
-bool knockout_plot_text(int x, int y, struct css_style *style,
+bool knockout_plot_text(int x, int y, const struct css_style *style,
 		const char *text, size_t length, colour bg, colour c)
 {
 	knockout_entries[knockout_entry_cur].data.text.x = x;
diff --git a/desktop/plotters.h b/desktop/plotters.h
index c0515e31b..79eac8170 100644
--- a/desktop/plotters.h
+++ b/desktop/plotters.h
@@ -40,8 +40,8 @@ struct plotter_table {
 	bool (*polygon)(int *p, unsigned int n, colour fill);
 	bool (*fill)(int x0, int y0, int x1, int y1, colour c);
 	bool (*clip)(int x0, int y0, int x1, int y1);
-	bool (*text)(int x, int y, struct css_style *style, const char *text,
-			size_t length, colour bg, colour c);
+	bool (*text)(int x, int y, const struct css_style *style,
+			const char *text, size_t length, colour bg, colour c);
 	bool (*disc)(int x, int y, int radius, colour c, bool filled);
 	bool (*arc)(int x, int y, int radius, int angle1, int angle2,
 	    		colour c);
diff --git a/gtk/gtk_plotters.c b/gtk/gtk_plotters.c
index 9f40f3694..24282c67d 100644
--- a/gtk/gtk_plotters.c
+++ b/gtk/gtk_plotters.c
@@ -57,7 +57,7 @@ static bool nsgtk_plot_path(float *p, unsigned int n, colour fill, float width,
 static bool nsgtk_plot_fill(int x0, int y0, int x1, int y1, colour c);
 static bool nsgtk_plot_clip(int clip_x0, int clip_y0,
 		int clip_x1, int clip_y1);
-static bool nsgtk_plot_text(int x, int y, struct css_style *style,
+static bool nsgtk_plot_text(int x, int y, const struct css_style *style,
 		const char *text, size_t length, colour bg, colour c);
 static bool nsgtk_plot_disc(int x, int y, int radius, colour c, bool filled);
 static bool nsgtk_plot_arc(int x, int y, int radius, int angle1, int angle2,
@@ -223,7 +223,7 @@ bool nsgtk_plot_clip(int clip_x0, int clip_y0,
 }
 
 
-bool nsgtk_plot_text(int x, int y, struct css_style *style,
+bool nsgtk_plot_text(int x, int y, const struct css_style *style,
 		const char *text, size_t length, colour bg, colour c)
 {
 	return nsfont_paint(style, text, length, x, y, c);
diff --git a/riscos/font.c b/riscos/font.c
index 0898175dc..d320b2070 100644
--- a/riscos/font.c
+++ b/riscos/font.c
@@ -368,7 +368,7 @@ bool nsfont_split(const struct css_style *style,
  * \return  true on success, false on error and error reported
  */
 
-bool nsfont_paint(struct css_style *style, const char *string,
+bool nsfont_paint(const struct css_style *style, const char *string,
 		size_t length, int x, int y, float scale)
 {
 	const char *font_family;
diff --git a/riscos/gui.h b/riscos/gui.h
index ee7ed896f..2c7d024b2 100644
--- a/riscos/gui.h
+++ b/riscos/gui.h
@@ -208,7 +208,7 @@ void ro_gui_print_prepare(struct gui_window *g);
 void nsfont_init(void);
 bool nsfont_exists(const char *font_family);
 const char *nsfont_fallback_font(void);
-bool nsfont_paint(struct css_style *style, const char *string,
+bool nsfont_paint(const struct css_style *style, const char *string,
 		size_t length, int x, int y, float scale);
 void nsfont_read_style(const struct css_style *style,
 		const char **font_family, unsigned int *font_size,
diff --git a/riscos/plotters.c b/riscos/plotters.c
index 1044df3a4..da890abad 100644
--- a/riscos/plotters.c
+++ b/riscos/plotters.c
@@ -46,7 +46,7 @@ static bool ro_plot_path(float *p, unsigned int n, colour fill, float width,
 static bool ro_plot_fill(int x0, int y0, int x1, int y1, colour c);
 static bool ro_plot_clip(int clip_x0, int clip_y0,
 		int clip_x1, int clip_y1);
-static bool ro_plot_text(int x, int y, struct css_style *style,
+static bool ro_plot_text(int x, int y, const struct css_style *style,
 		const char *text, size_t length, colour bg, colour c);
 static bool ro_plot_disc(int x, int y, int radius, colour colour, bool filled);
 static bool ro_plot_arc(int x, int y, int radius, int angle1, int angle2,
@@ -395,7 +395,7 @@ bool ro_plot_clip(int clip_x0, int clip_y0,
 }
 
 
-bool ro_plot_text(int x, int y, struct css_style *style,
+bool ro_plot_text(int x, int y, const struct css_style *style,
 		const char *text, size_t length, colour bg, colour c)
 {
 	os_error *error;
diff --git a/riscos/print.c b/riscos/print.c
index 411e80a15..66d1774e1 100644
--- a/riscos/print.c
+++ b/riscos/print.c
@@ -106,7 +106,7 @@ static bool print_fonts_plot_polygon(int *p, unsigned int n, colour fill);
 static bool print_fonts_plot_fill(int x0, int y0, int x1, int y1, colour c);
 static bool print_fonts_plot_clip(int clip_x0, int clip_y0,
 		int clip_x1, int clip_y1);
-static bool print_fonts_plot_text(int x, int y, struct css_style *style,
+static bool print_fonts_plot_text(int x, int y, const struct css_style *style,
 		const char *text, size_t length, colour bg, colour c);
 static bool print_fonts_plot_disc(int x, int y, int radius, colour c,
                 bool filled);
@@ -843,7 +843,7 @@ bool print_fonts_plot_group_end(void) { return true; }
  * Plotter for text plotting during font listing.
  */
 
-bool print_fonts_plot_text(int x, int y, struct css_style *style,
+bool print_fonts_plot_text(int x, int y, const struct css_style *style,
 		const char *text, size_t length, colour bg, colour c)
 {
 	const char *font_family;