Use border-box sizing for the HR element.

LayoutTests/platform/linux/fast/block/float/float-avoidance-expected.txt

 layer at (0,0) size 800x600 clip at (0,0) size 785x600 scrollHeight 2414
   LayoutView at (0,0) size 800x600
 layer at (0,0) size 785x2414 backgroundClip at (0,0) size 785x600 clip at (0,0) size 785x600 outlineClip at (0,0) size 785x600
   LayoutBlockFlow {HTML} at (0,0) size 785x2414
     LayoutBlockFlow {BODY} at (8,8) size 769x2398
       LayoutBlockFlow (anonymous) at (0,0) size 769x40
         LayoutText {#text} at (0,0) size 754x39
           text run at (0,0) width 517: "Test of objects that avoid floats to see what they do with percentage and auto widths. "
           text run at (517,0) width 237: "This test is designed to illustrate that we"
           text run at (0,20) width 402: "have removed the WinIE quirk and are behaving more like Firefox."
@@ skipping 230 matching lines @@
           text run at (0,0) width 420: "The floating hr with a percentage width should be even with the select."
       LayoutBlockFlow {DIV} at (0,2050) size 220x60 [border: (10px solid #FF0000)]
         LayoutText {#text} at (10,10) size 56x19
           text run at (10,10) width 56: "Line One"
         LayoutBR {BR} at (66,25) size 0x0
         LayoutMenuList {SELECT} at (10,30) size 100x20 [bgcolor=#C0C0C0] [border: (1px solid #A9A9A9)]
           LayoutBlockFlow (anonymous) at (1,1) size 98x18
             LayoutText at (4,1) size 24x16
               text run at (4,1) width 24: "One"
         LayoutText {#text} at (0,0) size 0x0
-        LayoutBlockFlow (floating) {HR} at (112,32) size 82x2 [border: (1px inset #EEEEEE)]
+        LayoutBlockFlow (floating) {HR} at (112,32) size 80x2 [border: (1px inset #EEEEEE)]
         LayoutText {#text} at (0,0) size 0x0
-        LayoutBR {BR} at (196,30) size 0x19
+        LayoutBR {BR} at (194,30) size 0x19
       LayoutBlockFlow (anonymous) at (0,2110) size 769x20
         LayoutText {#text} at (0,0) size 735x19
           text run at (0,0) width 735: "The floating hr below should still be even with the select and shrinks to use its intrinsic width (which is basically like 1-2px)."
       LayoutBlockFlow {DIV} at (0,2130) size 220x60 [border: (10px solid #FF0000)]
         LayoutText {#text} at (10,10) size 56x19
           text run at (10,10) width 56: "Line One"
         LayoutBR {BR} at (66,25) size 0x0
         LayoutMenuList {SELECT} at (10,30) size 100x20 [bgcolor=#C0C0C0] [border: (1px solid #A9A9A9)]
           LayoutBlockFlow (anonymous) at (1,1) size 98x18
             LayoutText at (4,1) size 24x16
               text run at (4,1) width 24: "One"
         LayoutText {#text} at (0,0) size 0x0
         LayoutBlockFlow (floating) {HR} at (112,32) size 2x2 [border: (1px inset #EEEEEE)]
         LayoutText {#text} at (0,0) size 0x0
         LayoutBR {BR} at (116,30) size 0x19
       LayoutBlockFlow (anonymous) at (0,2190) size 769x20
         LayoutText {#text} at (0,0) size 529x19
           text run at (0,0) width 529: "The block-level hr below has a percentage width and should still be even with the select."
       LayoutBlockFlow {DIV} at (0,2210) size 220x90 [border: (10px solid #FF0000)]
         LayoutBlockFlow (anonymous) at (10,10) size 200x20
           LayoutText {#text} at (0,0) size 56x19
             text run at (0,0) width 56: "Line One"
           LayoutBR {BR} at (56,15) size 0x0
           LayoutMenuList {SELECT} at (0,20) size 100x20 [bgcolor=#C0C0C0] [border: (1px solid #A9A9A9)]
             LayoutBlockFlow (anonymous) at (1,1) size 98x18
               LayoutText at (4,1) size 24x16
                 text run at (4,1) width 24: "One"
           LayoutText {#text} at (0,0) size 0x0
-        LayoutBlockFlow {HR} at (10,50) size 202x2 [border: (1px inset #EEEEEE)]
+        LayoutBlockFlow {HR} at (10,50) size 200x2 [border: (1px inset #EEEEEE)]
         LayoutBlockFlow (anonymous) at (10,60) size 200x20
           LayoutBR {BR} at (0,0) size 0x19
       LayoutBlockFlow (anonymous) at (0,2300) size 769x20
         LayoutText {#text} at (0,0) size 590x19
           text run at (0,0) width 500: "The block-level hr below has an auto width and should still be even with the select. "
           text run at (500,0) width 90: "It shrinks to fit."
       LayoutBlockFlow {DIV} at (0,2320) size 220x78 [border: (10px solid #FF0000)]
         LayoutBlockFlow (anonymous) at (10,10) size 200x20
           LayoutText {#text} at (0,0) size 56x19
             text run at (0,0) width 56: "Line One"
@@ skipping 29 matching lines @@
       text run at (0,60) width 95: "to have to wrap"
       text run at (0,80) width 96: "to multiple lines."
 layer at (118,1908) size 100x100 backgroundClip at (0,0) size 0x0 clip at (0,0) size 0x0 outlineClip at (0,0) size 785x600
   LayoutBlockFlow {DIV} at (110,30) size 100x100
     LayoutText {#text} at (0,0) size 99x99
       text run at (0,0) width 56: "This is an"
       text run at (0,20) width 99: "overflow section"
       text run at (0,40) width 98: "with enough text"
       text run at (0,60) width 95: "to have to wrap"
       text run at (0,80) width 96: "to multiple lines."