rem (root em) units

Type of unit similar to em, but relative only to the root element, not any parent element. Thus compounding does not occur as it does with em units.

Spec	https://www.w3.org/TR/css3-values/#font-relative-lengths
Status	W3C Candidate Recommendation

IE	Edge	Firefox	Chrome	Safari	Opera
			98
		95	97
		94	96	TP
11	95	93	95	15	80
10 (1)	94	92	94	14.1	79
9 (1)	93	91	93	14	78
8	92	90	92	13.1	77
Show all
7	91	89	91	13	76
6	90	88	90	12.1	75
5.5	89	87	89	12	74
	88	86	88	11.1	73
	87	85	87	11	72
	86	84	86	10.1	71
	85	83	85	10	70
	84	82	84	9.1	69
	83	81	83	9	68
	81	80	81	8	67
	80	79	80	7.1	66
	79	78	79	7	65
	18	77	78	6.1	64
	17	76	77	6	63
	16	75	76	5.1	62
	15	74	75	5	60
	14	73	74	4	58
	13	72	73	3.2	57
	12	71	72	3.1	56
		70	71		55
		69	70		54
		68	69		53
		67	68		52
		66	67		51
		65	66		50
		64	65		49
		63	64		48
		62	63		47
		61	62		46
		60	61		45
		59	60		44
		58	59		43
		57	58		42
		56	57		41
		55	56		40
		54	55		39
		53	54		38
		52	53		37
		51	52		36
		50	51		35
		49	50		34
		48	49		33
		47	48		32
		46	47		31
		45	46		30
		44	45		29
		43	44		28
		42	43		27
		41	42		26
		40	41		25
		39	40		24
		38	39		23
		37	38		22
		36	37		21
		35	36		20
		34	35		19
		33	34		18
		32	33		17
		31	32		16
		30	31		15
		29	30		12.1
		28	29		12
		27	28		11.6
		26	27		11.5
		25	26		11.1
		24	25		11
		23	24		10.6
		22	23		10.5
		21	22		10.0-10.1
		20	21		9.5-9.6
		19	20		9
		18	19
		17	18
		16	17
		15	16
		14	15
		13	14
		12	13
		11	12
		10	11
		9	10
		8	9
		7	8
		6	7
		5	6
		4	5
		3.6	4
		3.5
		3
		2

Safari on iOS	Opera Mini	Android Browser	Blackberry Browser	Opera Mobile	Android Chrome	Android Firefox	IE Mobile	Android UC Browser	Samsung Internet	QQ Browser	Baidu Browser	KaiOS Browser
15	all	94	10	64	94	92	11	12.12	15.0	10.4	7.12	2.5
14.5-14.8		4.4.3-4.4.4	7	12.1			10		14.0
14.0-14.4		4.4		12					13.0
13.4-13.7		4.2-4.3		11.5					12.0
Show all
13.3		4.1		11.1					11.1-11.2
13.2		4		11					10.1
13.0-13.1		3		10					9.2
12.2-12.5		2.3							8.2
12.0-12.1		2.2							7.2-7.4
11.3-11.4		2.1							6.2-6.4
11.0-11.2									5.0-5.4
10.3									4
10.0-10.2
9.3
9.0-9.2
8.1-8.4
8
7.0-7.1
6.0-6.1
5.0-5.1 (2)
4.2-4.3
4.0-4.1
3.2

Notes

1. IE 9 & IE 10 do not support rem units when used in the font shorthand property (the entire declaration is ignored) or when used on pseudo elements.

2. iOS Safari 5.0-5.1 support rem but not in combination with media queries.

Bugs

• IE 9, 10 and 11 do not support rem units when used in the "line-height" property when used on :before and :after pseudo elements (https://web.archive.org/web/20160801002559/https://connect.microsoft.com/IE/feedback/details/776744).

• Borders sized in "rem" disappear when the page is zoomed out in Chrome.

• Reportedly does not work on Android 4.3 browser for Samsung Note II or the Samsung Galaxy Tab 2 on Android 4.2.

• Chrome 31-34 & Chrome-based Android versions (like 4.4) have a font size bug that occurs when the root element has a percentage-based size.

• Causes content display and scrolling issues on iPhone 4 which typically has Safari 5.1.

Resources

• Article on usage
• REM Polyfill