arranging div one below the other

Question

Try a clear: left on #inner2. Because they are both being set to float it should cause a line return.

#inner1 {
   float:left; 
}

#inner2{
   float:left; 
   clear: left;
}

<div id="wrapper">
    <div id="inner1">This is inner div 1</div>
    <div id="inner2">This is inner div 2</div>
</div>

If you want the two divs to be displayed one above the other, the simplest answer is to remove the float: left;from the css declaration, as this causes them to collapse to the size of their contents (or the css defined size), and, well float up against each other.

Alternatively, you could simply add clear:both; to the divs, which will force the floated content to clear previous floats.

The default behaviour of divs is to take the full width available in their parent container.
This is the same as if you'd give the inner divs a width of 100%.

By floating the divs, they ignore their default and size their width to fit the content. Everything behind it (in the HTML), is placed under the div (on the rendered page).
This is the reason that they align theirselves next to each other.

The float CSS property specifies that an element should be taken from the normal flow and placed along the left or right side of its container, where text and inline elements will wrap around it. A floating element is one where the computed value of float is not none.

Source: https://developer.mozilla.org/en-US/docs/Web/CSS/float

Get rid of the float, and the divs will be aligned under each other.
If this does not happen, you'll have some other css on divs or children of wrapper defining a floating behaviour or an inline display.

If you want to keep the float, for whatever reason, you can use clear on the 2nd div to reset the floating properties of elements before that element.
clear has 5 valid values: none | left | right | both | inherit. Clearing no floats (used to override inherited properties), left or right floats or both floats. Inherit means it'll inherit the behaviour of the parent element

Also, because of the default behaviour, you don't need to set the width and height on auto.
You only need this is you want to set a hardcoded height/width. E.g. 80% / 800px / 500em / ...

<div id="wrapper">
    <div id="inner1"></div>
    <div id="inner2"></div>
</div>

CSS is

#wrapper{
    margin-left:auto;
    margin-right:auto;
    height:auto; // this is not needed, as parent container, this div will size automaticly
    width:auto; // this is not needed, as parent container, this div will size automaticly
}

/*
You can get rid of the inner divs in the css, unless you want to style them.
If you want to style them identicly, you can use concatenation
*/
#inner1, #inner2 {
    border: 1px solid black;
}

Set the main div CSS to somthing like:

<style>
    .wrapper{
        display:flex;
        flex-direction: column;
    }
</style>

<div id="wrapper">
        <div id="inner1">This is inner div 1</div>
        <div id="inner2">This is inner div 2</div>
</div>

For more flexbox CSS refer: https://css-tricks.com/snippets/css/a-guide-to-flexbox/

You don't even need the float:left;

It seems the default behavior is to render one below the other, if it doesn't happen it's because they are inheriting some style from above.

CSS:

#wrapper{
    margin-left:auto;
    margin-right:auto;
    height:auto; 
    width:auto;
}
</style>

HTML:

<div id="wrapper">
    <div id="inner1">inner1</div>
    <div id="inner2">inner2</div>
</div>