Layout changed when I added spacing around images.

Question

@charset "utf-8";
body {
font: 100%/1.4 Verdana, Arial, Helvetica, sans-serif;
background: #363;
margin: 0;
padding: 0;
color: #000;
/* ~~ Element/tag selectors ~~ */
ul, ol, dl { /* Due to variations between browsers, it's best practices to zero padding and margin on lists. For consistency, you can either specify the amounts you want here, or on the list items (LI, DT, DD) they contain. Remember that what you do here will cascade to the .nav list unless you write a more specific selector. */
padding: 0;
margin: 0;
h1, h2, h3, h4, h5, h6, p {
margin-top: 0;  /* removing the top margin gets around an issue where margins can escape from their containing div. The remaining bottom margin will hold it away from any elements that follow. */
padding-right: 15px;
padding-left: 15px; /* adding the padding to the sides of the elements within the divs, instead of the divs themselves, gets rid of any box model math. A nested div with side padding can also be used as an alternate method. */
a img { /* this selector removes the default blue border displayed in some browsers around an image when it is surrounded by a link */
border: none;
/* ~~ Styling for your site's links must remain in this order - including the group of selectors that create the hover effect. ~~ */
a:link {
color:#414958;
text-decoration: underline; /* unless you style your links to look extremely unique, it's best to provide underlines for quick visual identification */
a:visited {
color: #F00;
text-decoration: underline;
a:hover, a:active, a:focus { /* this group of selectors will give a keyboard navigator the same hover experience as the person using a mouse. */
text-decoration: none;
/* ~~ this container surrounds all other divs giving them their percentage-based width ~~ */
.container {
width: 80%;
max-width: 1260px;/* a max-width may be desirable to keep this layout from getting too wide on a large monitor. This keeps line length more readable. IE6 does not respect this declaration. */
min-width: 780px;/* a min-width may be desirable to keep this layout from getting too narrow. This keeps line length more readable in the side columns. IE6 does not respect this declaration. */
background: #FFF;
margin: 0 auto; /* the auto value on the sides, coupled with the width, centers the layout. It is not needed if you set the .container's width to 100%. */
/* ~~ the header is not given a width. It will extend the full width of your layout. It contains an image placeholder that should be replaced with your own linked logo ~~ */
.header {
background: #363;
/* ~~ These are the columns for the layout. ~~
1) Padding is only placed on the top and/or bottom of the divs. The elements within these divs have padding on their sides. This saves you from any "box model math". Keep in mind, if you add any side padding or border to the div itself, it will be added to the width you define to create the *total* width. You may also choose to remove the padding on the element in the div and place a second div within it with no width and the padding necessary for your design.
2) No margin has been given to the columns since they are all floated. If you must add margin, avoid placing it on the side you're floating toward (for example: a right margin on a div set to float right). Many times, padding can be used instead. For divs where this rule must be broken, you should add a "display:inline" declaration to the div's rule to tame a bug where some versions of Internet Explorer double the margin.
3) Since classes can be used multiple times in a document (and an element can also have multiple classes applied), the columns have been assigned class names instead of IDs. For example, two sidebar divs could be stacked if necessary. These can very easily be changed to IDs if that's your preference, as long as you'll only be using them once per document.
4) If you prefer your nav on the right instead of the left, simply float these columns the opposite direction (all right instead of all left) and they'll render in reverse order. There's no need to move the divs around in the HTML source.
.sidebar1 {
float: left;
width: 20%;
background: #CF0;
padding-bottom: 10px;
.content {
padding: 15px 15px 15px 15px;
width: 80%;
float: left;
/* ~~ This grouped selector gives the lists in the .content area space ~~ */
.content ul, .content ol {
padding: 0 15px 15px 40px; /* this padding mirrors the right padding in the headings and paragraph rule above. Padding was placed on the bottom for space between other elements on the lists and on the left to create the indention. These may be adjusted as you wish. */
/* ~~ The navigation list styles (can be removed if you choose to use a premade flyout menu like Spry) ~~ */
ul.nav {
list-style: none; /* this removes the list marker */
border-top: 1px solid #666; /* this creates the top border for the links - all others are placed using a bottom border on the LI */
margin-bottom: 15px; /* this creates the space between the navigation on the content below */
ul.nav li {
border-bottom: 1px solid #666; /* this creates the button separation */
ul.nav a, ul.nav a:visited { /* grouping these selectors makes sure that your links retain their button look even after being visited */
padding: 5px 5px 5px 15px;
display: block; /* this gives the link block properties causing it to fill the whole LI containing it. This causes the entire area to react to a mouse click. */
text-decoration: none;
background: #390;
color: #000;
ul.nav a:hover, ul.nav a:active, ul.nav a:focus { /* this changes the background and text color for both mouse and keyboard navigators */
background: #6F7D94;
color: #FFF;
/* ~~ The footer ~~ */
.footer {
padding: 10px 0;
background: #CF6;
position: relative;/* this gives IE6 hasLayout to properly clear */
clear: both; /* this clear property forces the .container to understand where the columns end and contain them */
/* ~~ miscellaneous float/clear classes ~~ */
.fltrt {  /* this class can be used to float an element right in your page. The floated element must precede the element it should be next to on the page. */
float: right;
margin-left: 8px;
.fltlft { /* this class can be used to float an element left in your page. The floated element must precede the element it should be next to on the page. */
float: left;
margin-right: 8px;
.clearfloat { /* this class can be placed on a <br /> or empty div as the final element following the last floated div (within the #container) if the #footer is removed or taken out of the #container */
clear:both;
height:0;
font-size: 1px;
line-height: 0px;
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Yellowstone Biologist</title>
<link href="NationalParks.css" rel="stylesheet" type="text/css" /><!--[if lte IE 7]>
<style>
.content { margin-right: -1px; } /* this 1px negative margin can be placed on any of the columns in this layout with the same corrective effect. */
ul.nav a { zoom: 1; }  /* the zoom property gives IE the hasLayout trigger it needs to correct extra whiltespace between the links */
</style>
<![endif]-->
<script type="text/javascript">
</script>
</head>
<body onload="MM_preloadImages('habitat def.PNG')">
<div class="container">
  <div class="header"><a href= /><img src="animals.gif" width="878" height="193" alt="bison" /></a>
    <!-- end .header --></div>
  <div class="sidebar1">
    <ul class="nav">
      <li><a href="NationalParks.html"><strong>Home</strong></a></li>
      <li><a href="Park Ranger.html"><strong>Park Ranger</strong></a></li>
      <li><a href="Biologist.html"><strong>Biologist</strong></a></li>
      <li><a href="Geologist.html"><strong>Geologist</strong></a></li>
      <li><a href="Historian.html"><strong>Historian</strong></a></li>
      <li><a href="Games.html"><strong>Games</strong></a></li>
    </ul>
    <p><strong>Definitions (italicized words):</strong> </p>
    <p><em>Habitat</em>:   the natural conditions and environment  in which a plant or animal lives</p>
    <p><br />
      <em>Extermination</em>:  completely destroy or kill</p>
    <p><em>USFWS</em>: United States Fish and Wildlife Service</p>
<!-- end .sidebar1 --></div>
  <div class="content">
    <h1>Yellowstone Biologist    </h1>
    <h2>Mammals</h2>
   <p> Yellowstone is home to the largest concentration of mammals in the lower 48 states. Sixty-seven different mammals live here, including grizzly bears, black bears, gray wolves, wolverine, lynx, elk, bison, moose, and numerous small mammals.</p>
    <p>Visitors must view all wildlife safely. You must stay at least 100 yards away from bears and wolves and at least 25 yards  away from all other animals—including bison, elk, bighorn sheep, deer, moose, and coyotes.</p>
    <h4>Bears</h4><img src="grizzly.gif" alt="grizzly bear family" width="150" height="115" hspace="15" vspace="15" align="left" />
    <p>Bears may be seen in Yellowstone March through November of each year. Yellowstone is one of the only areas south of Canada that still has large grizzly bears. Grizzly bears are usually seen in the open areas. </p>
    <p> </p>
    <h4>Wolves</h4><img src="wolves.gif" alt="wolves" width="150" height="115" hspace="15" vspace="15" align="left" />
    <p>
Loss of <em>habitat</em> and <em>extermination</em> programs led to the elimination of wolves throughout most of the United States by early in the 20th century. In 1973, the U.S. Fish and Wildlife Service listed the northern Rocky Mountain wolf  as an endangered species and designated Greater Yellowstone as one of three recovery areas. From 1995 to 1997, 41 wild wolves from Canada and northwest Montana were released in Yellowstone National Park. As expected, wolves from the growing population dispersed to establish territories outside the park. More than 1,600 wolves now reside in Idaho, Montana, and Wyoming. In August 2010, a U.S. district judge ruled against a <em>USFWS</em> 2009 decision to remove the wolf from the endangered species list, so wolves are protected as an endangered species in all three states again.<br />
    </p>
<h4>Bighorn Sheep<img src="BigHornSheep.gif" alt="bighorn sheep" width="150" height="115" hspace="15" vspace="15" align="left" /></h4>
    <p> Bighorn sheep once numbered in the millions in western United States and were an important food source for humans. They are named for the large, curved horns on the males, or rams. Females, or ewes, also have horns, but they are short with only a slight curve. In the winter of 1981-82, an outbreak of pinkeye occurred among bighorns in the Mt. Everts area. Many sheep were blinded and/or killed on the adjacent park road or by falling from cliffs. Some are now found comfortable around summer road traffic.</p>
    <h4>Bison<img src="bison.gif" alt="bison" width="150" height="115" hspace="15" vspace="15" align="left" /></h4><p>
    Bison are the largest mammals in Yellowstone National Park. They are strictly vegetarian, and they graze on grasslands in the meadows, the foothills, and even the high-elevation areas of Yellowstone. Despite their slow walk, bison are surprisingly fast for animals that weigh more than half a ton.</p>
<p>Yellowstone is the only place in the lower 48 states where a population of wild bison has survived since prehistoric times, although fewer than 50 native bison remained here in 1902. Fearing extinction, the park imported 21 bison from two privately-owned herds.</p>
    <h4>Bobcats<img src="yellowstone-bobcat.gif" alt="bobcat" width="150" height="115" hspace="15" vspace="15" align="left" /></h4><p>
    Bobcats  are small wild cats with reddish-brown or yellowish-brown coats, streaked with black or dark brown. They have prominent, pointed ears with a tuft of black hair at the tip. </p>
<p>Unlike lynx, which they resemble, bobcats elsewhere have been highly adaptable to human-caused changes in environmental conditions; some biologists believe that there are more bobcats in the United States today than in colonial times. Yellowstone has many rock outcrops, canyons bordered by rock ledges, conifer forests, and semi-open areas that seem to offer conditions favorable for bobcats--adequate shelter, a variety of rodents, rabbits, hares, birds, and other small animals as well as seasonal carrion, for food.</p>
<p>Bobcats are known to hole-up and wait out severe winter storms elsewhere. These elusive cats are most active at night, so even those who study them seldom have an opportunity to see one.</p>
    <h4>Coyotes</h4>
    <p>
      Yellowstone's coyotes  are among the largest coyotes in the United States. Adults are about 30 pounds, are less than two feet tall and vary in color from gray to tan with sometimes a reddish tint to its coat.</p>
    <p>Small mammals are an important component of coyotes' diets. In Yellowstone, such prey include microtines or voles, mice , pocket gophers, small birds, and in spring and summer, ground squirrels. They also feed on larger mammal's (elk, deer, bison, moose, pronghorn, and bighorn sheep) calves and fawns shortly after birth and young animals.</p>
    <h4>Elk<img src="elk.gif" alt="elk" width="150" height="115" hspace="15" vspace="15" align="left" /></h4><p>
      Elk  are the most abundant large mammal found in Yellowstone and historical evidence confirms their continuous presence for at least 1,000 years. More than 30,000 elk from 7-8 different herds summer in Yellowstone and approximately 15,000 to 22,000 winter in the park. </p>
<p>Bulls grow antlers annually from the time they are nearly one year old. When mature, a bull's &quot;rack&quot; may have 6 to 8 points, or tines, on each side and weigh more than 30 pounds.</p>
    <h4>Lynx</h4>
<p>Adult lynx are about the size of a large domestic cat. Males can weigh up to 30 pounds, while females are smaller. The similarity between lynx and bobcats makes it difficult to determine their status in Yellowstone. A large adult bobcat may be larger than a small adult lynx, so size is not a good characteristic for positive identification.</p>
    <h4>Moose<img src="moose.gif" alt="moose" width="150" height="115" hspace="15" vspace="15" align="left" /></h4><p>
    Moose, the largest member of the deer family, were reportedly very rare in northwest Wyoming when Yellowstone National Park was established in 1872. Moose breed from early September to November and one to three calves are born in May or June. Calves weigh 25 to 35 pounds at birth but grow rapidly; adult females (cows) weigh up to 800 pounds and males (bulls) up to 1300 pounds. </p>
<h4>Mountain Lion</h4><p>
    The mountain lion, also called the cougar, is the largest member of the cat family living in Yellowstone. Mountain lions can weigh up to 200 pounds, although lions in Yellowstone are thought to range between 140 and 160 pounds for males and around 100 pounds for females.</p>
<p>Mountain lions are rather secretive, so most visitors are unaware of their existence in Yellowstone. Lions probably live throughout the park in summer. </p>
    <h4>      Amphibians</h4>
    <p>
    Cool, dry conditions limit Yellowstone's amphibians to four species: boreal toad; chorus frog; spotted frog; and the tiger salamander. Population numbers for these  are not known.</p>
    <!-- end .content --></div>
  <div class="footer">
    <p>Sources: <a href="http://www.nps.gov/yell/naturescience/mammals.htm">http://www.nps.gov/yell/naturescience/m ammals.htm</a></p>
    <p><a href="http://www.nps.gov/yell/naturescience/amphibians.htm">http://www.nps.gov/yell/naturescienc e/amphibians.htm</a></p>
    <!-- end .footer --></div>
  <!-- end .container --></div>
</body>
</html>
This has happened before, where everything in my content area has dropped down below the sidebar.It's happended on several pages. I don't know why or how to fix it. It's for a class, so where I've posted it is private, but here's the page and css code.
HTML

Answer 1

Murray is right. Padding (and margins) is added to the width. However, when using percentages, you should never use figures that add up to exactly 100%.
Browsers need to convert the percentages to pixels. Because pixels must be whole numbers, some percentages are rounded up, which results in the final element dropping down below its neighbours. With percentages, it's much safer to use values that add up to 98% (99% often works, but 98% is safer).